diff --git a/src/applications/conpherence/controller/ConpherenceViewController.php b/src/applications/conpherence/controller/ConpherenceViewController.php
index 6b953b9de4..7a152a1e2e 100644
--- a/src/applications/conpherence/controller/ConpherenceViewController.php
+++ b/src/applications/conpherence/controller/ConpherenceViewController.php
@@ -1,244 +1,241 @@
 <?php
 
 final class ConpherenceViewController extends
   ConpherenceController {
 
   const OLDER_FETCH_LIMIT = 5;
 
   public function shouldAllowPublic() {
     return true;
   }
 
   public function handleRequest(AphrontRequest $request) {
     $user = $request->getUser();
 
     $conpherence_id = $request->getURIData('id');
     if (!$conpherence_id) {
       return new Aphront404Response();
     }
     $query = id(new ConpherenceThreadQuery())
       ->setViewer($user)
       ->withIDs(array($conpherence_id))
       ->needProfileImage(true)
       ->needTransactions(true)
       ->setTransactionLimit($this->getMainQueryLimit());
 
     $before_transaction_id = $request->getInt('oldest_transaction_id');
     $after_transaction_id = $request->getInt('newest_transaction_id');
     $old_message_id = $request->getURIData('messageID');
     if ($before_transaction_id && ($old_message_id || $after_transaction_id)) {
       throw new Aphront400Response();
     }
     if ($old_message_id && $after_transaction_id) {
       throw new Aphront400Response();
     }
 
     $marker_type = 'older';
     if ($before_transaction_id) {
       $query
         ->setBeforeTransactionID($before_transaction_id);
     }
     if ($old_message_id) {
       $marker_type = 'olderandnewer';
       $query
         ->setAfterTransactionID($old_message_id - 1);
     }
     if ($after_transaction_id) {
       $marker_type = 'newer';
       $query
         ->setAfterTransactionID($after_transaction_id);
     }
 
     $conpherence = $query->executeOne();
     if (!$conpherence) {
       return new Aphront404Response();
     }
     $this->setConpherence($conpherence);
 
     $transactions = $this->getNeededTransactions(
       $conpherence,
       $old_message_id);
     $latest_transaction = head($transactions);
     $participant = $conpherence->getParticipantIfExists($user->getPHID());
     if ($participant) {
       if (!$participant->isUpToDate($conpherence)) {
         $write_guard = AphrontWriteGuard::beginScopedUnguardedWrites();
         $participant->markUpToDate($conpherence, $latest_transaction);
         $user->clearCacheData(PhabricatorUserMessageCountCacheType::KEY_COUNT);
         unset($write_guard);
       }
     }
 
     $data = ConpherenceTransactionRenderer::renderTransactions(
       $user,
       $conpherence,
       $marker_type);
     $messages = ConpherenceTransactionRenderer::renderMessagePaneContent(
       $data['transactions'],
       $data['oldest_transaction_id'],
       $data['newest_transaction_id']);
     if ($before_transaction_id || $after_transaction_id) {
       $header = null;
       $form = null;
       $content = array('transactions' => $messages);
     } else {
       $policy_objects = id(new PhabricatorPolicyQuery())
         ->setViewer($user)
         ->setObject($conpherence)
         ->execute();
       $header = $this->buildHeaderPaneContent($conpherence, $policy_objects);
       $search = $this->buildSearchForm();
       $form = $this->renderFormContent();
       $content = array(
         'header' => $header,
         'search' => $search,
         'transactions' => $messages,
         'form' => $form,
       );
     }
 
     $d_data = $conpherence->getDisplayData($user);
     $content['title'] = $title = $d_data['title'];
 
     if ($request->isAjax()) {
       $dropdown_query = id(new AphlictDropdownDataQuery())
         ->setViewer($user);
       $dropdown_query->execute();
       $content['threadID'] = $conpherence->getID();
       $content['threadPHID'] = $conpherence->getPHID();
       $content['latestTransactionID'] = $data['latest_transaction_id'];
       $content['canEdit'] = PhabricatorPolicyFilter::hasCapability(
         $user,
         $conpherence,
         PhabricatorPolicyCapability::CAN_EDIT);
       $content['aphlictDropdownData'] = array(
         $dropdown_query->getNotificationData(),
         $dropdown_query->getConpherenceData(),
       );
       return id(new AphrontAjaxResponse())->setContent($content);
     }
 
     $layout = id(new ConpherenceLayoutView())
       ->setUser($user)
       ->setBaseURI($this->getApplicationURI())
       ->setThread($conpherence)
       ->setHeader($header)
       ->setSearch($search)
       ->setMessages($messages)
       ->setReplyForm($form)
       ->setLatestTransactionID($data['latest_transaction_id'])
       ->setRole('thread');
 
     $participating = $conpherence->getParticipantIfExists($user->getPHID());
 
     if (!$user->isLoggedIn()) {
       $layout->addClass('conpherence-no-pontificate');
     }
 
     return $this->newPage()
       ->setTitle($title)
       ->setPageObjectPHIDs(array($conpherence->getPHID()))
       ->appendChild($layout);
   }
 
   private function renderFormContent() {
 
     $conpherence = $this->getConpherence();
     $user = $this->getRequest()->getUser();
 
     $participating = $conpherence->getParticipantIfExists($user->getPHID());
-    if (!$participating && $user->isLoggedIn()) {
-      return null;
-    }
     $draft = PhabricatorDraft::newFromUserAndKey(
       $user,
       $conpherence->getPHID());
     $update_uri = $this->getApplicationURI('update/'.$conpherence->getID().'/');
 
     if ($user->isLoggedIn()) {
       $this->initBehavior('conpherence-pontificate');
       if ($participating) {
         $action = ConpherenceUpdateActions::MESSAGE;
         $status = new PhabricatorNotificationStatusView();
       } else {
         $action = ConpherenceUpdateActions::JOIN_ROOM;
         $status = pht('Sending a message will also join the room.');
       }
 
       $form = id(new AphrontFormView())
         ->setUser($user)
         ->setAction($update_uri)
         ->addSigil('conpherence-pontificate')
         ->setWorkflow(true)
         ->addHiddenInput('action', $action)
         ->appendChild(
           id(new PhabricatorRemarkupControl())
           ->setUser($user)
           ->setName('text')
           ->setSendOnEnter(true)
           ->setValue($draft->getDraft()));
 
       $status_view = phutil_tag(
         'div',
         array(
           'class' => 'conpherence-room-status',
           'id' => 'conpherence-room-status',
         ),
         $status);
 
       $view = phutil_tag_div(
         'pontificate-container', array($form, $status_view));
 
       return $view;
 
     } else {
       // user not logged in so give them a login button.
       $login_href = id(new PhutilURI('/auth/start/'))
         ->setQueryParam('next', '/'.$conpherence->getMonogram());
       return id(new PHUIFormLayoutView())
         ->addClass('login-to-participate')
         ->appendInstructions(pht('Log in to join this room and participate.'))
         ->appendChild(
           id(new PHUIButtonView())
           ->setTag('a')
           ->setText(pht('Login to Participate'))
           ->setHref((string)$login_href));
     }
   }
 
   private function getNeededTransactions(
     ConpherenceThread $conpherence,
     $message_id) {
 
     if ($message_id) {
       $newer_transactions = $conpherence->getTransactions();
       $query = id(new ConpherenceTransactionQuery())
         ->setViewer($this->getRequest()->getUser())
         ->withObjectPHIDs(array($conpherence->getPHID()))
         ->setAfterID($message_id)
         ->needHandles(true)
         ->setLimit(self::OLDER_FETCH_LIMIT);
       $older_transactions = $query->execute();
       $handles = array();
       foreach ($older_transactions as $transaction) {
         $handles += $transaction->getHandles();
       }
       $conpherence->attachHandles($conpherence->getHandles() + $handles);
       $transactions = array_merge($newer_transactions, $older_transactions);
       $conpherence->attachTransactions($transactions);
     } else {
       $transactions = $conpherence->getTransactions();
     }
 
     return $transactions;
   }
 
   private function getMainQueryLimit() {
     $request = $this->getRequest();
     $base_limit = ConpherenceThreadQuery::TRANSACTION_LIMIT;
     if ($request->getURIData('messageID')) {
       $base_limit = $base_limit - self::OLDER_FETCH_LIMIT;
     }
     return $base_limit;
   }
 }
diff --git a/src/applications/conpherence/editor/ConpherenceEditor.php b/src/applications/conpherence/editor/ConpherenceEditor.php
index e358700348..c53aab7655 100644
--- a/src/applications/conpherence/editor/ConpherenceEditor.php
+++ b/src/applications/conpherence/editor/ConpherenceEditor.php
@@ -1,517 +1,527 @@
 <?php
 
 final class ConpherenceEditor extends PhabricatorApplicationTransactionEditor {
 
   const ERROR_EMPTY_PARTICIPANTS = 'error-empty-participants';
   const ERROR_EMPTY_MESSAGE = 'error-empty-message';
 
   public function getEditorApplicationClass() {
     return 'PhabricatorConpherenceApplication';
   }
 
   public function getEditorObjectsDescription() {
     return pht('Conpherence Rooms');
   }
 
   public static function createThread(
     PhabricatorUser $creator,
     array $participant_phids,
     $title,
     $message,
     PhabricatorContentSource $source,
     $topic) {
 
     $conpherence = ConpherenceThread::initializeNewRoom($creator);
     $errors = array();
     if (empty($participant_phids)) {
       $errors[] = self::ERROR_EMPTY_PARTICIPANTS;
     } else {
       $participant_phids[] = $creator->getPHID();
       $participant_phids = array_unique($participant_phids);
     }
 
     if (empty($message)) {
       $errors[] = self::ERROR_EMPTY_MESSAGE;
     }
 
     if (!$errors) {
       $xactions = array();
       $xactions[] = id(new ConpherenceTransaction())
         ->setTransactionType(ConpherenceTransaction::TYPE_PARTICIPANTS)
         ->setNewValue(array('+' => $participant_phids));
       if ($title) {
         $xactions[] = id(new ConpherenceTransaction())
           ->setTransactionType(
             ConpherenceThreadTitleTransaction::TRANSACTIONTYPE)
           ->setNewValue($title);
       }
       if (strlen($topic)) {
         $xactions[] = id(new ConpherenceTransaction())
           ->setTransactionType(
             ConpherenceThreadTopicTransaction::TRANSACTIONTYPE)
           ->setNewValue($topic);
       }
 
       $xactions[] = id(new ConpherenceTransaction())
         ->setTransactionType(PhabricatorTransactions::TYPE_COMMENT)
         ->attachComment(
           id(new ConpherenceTransactionComment())
           ->setContent($message)
           ->setConpherencePHID($conpherence->getPHID()));
 
       id(new ConpherenceEditor())
         ->setActor($creator)
         ->setContentSource($source)
         ->setContinueOnNoEffect(true)
         ->applyTransactions($conpherence, $xactions);
     }
 
     return array($errors, $conpherence);
   }
 
   public function generateTransactionsFromText(
     PhabricatorUser $viewer,
     ConpherenceThread $conpherence,
     $text) {
 
     $xactions = array();
     $xactions[] = id(new ConpherenceTransaction())
       ->setTransactionType(PhabricatorTransactions::TYPE_COMMENT)
       ->attachComment(
         id(new ConpherenceTransactionComment())
         ->setContent($text)
         ->setConpherencePHID($conpherence->getPHID()));
     return $xactions;
   }
 
   public function getTransactionTypes() {
     $types = parent::getTransactionTypes();
 
     $types[] = ConpherenceTransaction::TYPE_PARTICIPANTS;
 
     $types[] = PhabricatorTransactions::TYPE_COMMENT;
     $types[] = PhabricatorTransactions::TYPE_VIEW_POLICY;
     $types[] = PhabricatorTransactions::TYPE_EDIT_POLICY;
 
     return $types;
   }
 
   public function getCreateObjectTitle($author, $object) {
     return pht('%s created this room.', $author);
   }
 
   protected function getCustomTransactionOldValue(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     switch ($xaction->getTransactionType()) {
       case ConpherenceTransaction::TYPE_PARTICIPANTS:
         if ($this->getIsNewObject()) {
           return array();
         }
         return $object->getParticipantPHIDs();
     }
   }
 
   protected function getCustomTransactionNewValue(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     switch ($xaction->getTransactionType()) {
       case ConpherenceTransaction::TYPE_PARTICIPANTS:
         return $this->getPHIDTransactionNewValue($xaction);
     }
   }
 
   /**
    * We really only need a read lock if we have a comment. In that case, we
    * must update the messagesCount field on the conpherence and
    * seenMessagesCount(s) for the participant(s).
    */
   protected function shouldReadLock(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     $lock = false;
     switch ($xaction->getTransactionType()) {
       case PhabricatorTransactions::TYPE_COMMENT:
         $lock =  true;
         break;
     }
 
     return $lock;
   }
 
   /**
    * We need to apply initial effects IFF the conpherence is new. We must
    * save the conpherence first thing to make sure we have an id and a phid, as
    * well as create the initial set of participants so that we pass policy
    * checks.
    */
   protected function shouldApplyInitialEffects(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     return $this->getIsNewObject();
   }
 
   protected function applyInitialEffects(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $object->save();
 
     foreach ($xactions as $xaction) {
       switch ($xaction->getTransactionType()) {
         case ConpherenceTransaction::TYPE_PARTICIPANTS:
           // Since this is a new ConpherenceThread, we have to create the
           // participation data asap to pass policy checks. For existing
           // ConpherenceThreads, the existing participation is correct
           // at this stage. Note that later in applyCustomExternalTransaction
           // this participation data will be updated, particularly the
           // behindTransactionPHID which is just a generated dummy for now.
           $participants = array();
           $phids = $this->getPHIDTransactionNewValue($xaction, array());
           foreach ($phids as $phid) {
             if ($phid == $this->getActor()->getPHID()) {
               $status = ConpherenceParticipationStatus::UP_TO_DATE;
               $message_count = 1;
             } else {
               $status = ConpherenceParticipationStatus::BEHIND;
               $message_count = 0;
             }
             $participants[$phid] =
               id(new ConpherenceParticipant())
               ->setConpherencePHID($object->getPHID())
               ->setParticipantPHID($phid)
               ->setParticipationStatus($status)
               ->setDateTouched(time())
               ->setBehindTransactionPHID($xaction->generatePHID())
               ->setSeenMessageCount($message_count)
               ->save();
             $object->attachParticipants($participants);
           }
           break;
       }
     }
   }
 
   protected function applyCustomInternalTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     switch ($xaction->getTransactionType()) {
       case ConpherenceTransaction::TYPE_PARTICIPANTS:
         if (!$this->getIsNewObject()) {}
         break;
     }
 
   }
 
   protected function applyBuiltinInternalTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     switch ($xaction->getTransactionType()) {
       case PhabricatorTransactions::TYPE_COMMENT:
         $object->setMessageCount((int)$object->getMessageCount() + 1);
         break;
     }
 
     return parent::applyBuiltinInternalTransaction($object, $xaction);
   }
 
   protected function applyCustomExternalTransaction(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     switch ($xaction->getTransactionType()) {
       case ConpherenceTransaction::TYPE_PARTICIPANTS:
         if ($this->getIsNewObject()) {
           continue;
         }
         $participants = $object->getParticipants();
 
         $old_map = array_fuse($xaction->getOldValue());
         $new_map = array_fuse($xaction->getNewValue());
 
         $remove = array_keys(array_diff_key($old_map, $new_map));
         foreach ($remove as $phid) {
           $remove_participant = $participants[$phid];
           $remove_participant->delete();
           unset($participants[$phid]);
         }
 
         $add = array_keys(array_diff_key($new_map, $old_map));
         foreach ($add as $phid) {
           if ($phid == $this->getActor()->getPHID()) {
             $status = ConpherenceParticipationStatus::UP_TO_DATE;
             $message_count = $object->getMessageCount();
           } else {
             $status = ConpherenceParticipationStatus::BEHIND;
             $message_count = 0;
           }
           $participants[$phid] =
             id(new ConpherenceParticipant())
             ->setConpherencePHID($object->getPHID())
             ->setParticipantPHID($phid)
             ->setParticipationStatus($status)
             ->setDateTouched(time())
             ->setBehindTransactionPHID($xaction->getPHID())
             ->setSeenMessageCount($message_count)
             ->save();
         }
         $object->attachParticipants($participants);
         break;
     }
   }
 
   protected function applyFinalEffects(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     if (!$xactions) {
       return $xactions;
     }
 
     $message_count = 0;
     foreach ($xactions as $xaction) {
       switch ($xaction->getTransactionType()) {
         case PhabricatorTransactions::TYPE_COMMENT:
           $message_count++;
 
           // update everyone's participation status on a message -only-
           $xaction_phid = $xaction->getPHID();
           $behind = ConpherenceParticipationStatus::BEHIND;
           $up_to_date = ConpherenceParticipationStatus::UP_TO_DATE;
           $participants = $object->getParticipants();
           $user = $this->getActor();
           $time = time();
           foreach ($participants as $phid => $participant) {
             if ($phid != $user->getPHID()) {
               if ($participant->getParticipationStatus() != $behind) {
                 $participant->setBehindTransactionPHID($xaction_phid);
                 $participant->setSeenMessageCount(
                   $object->getMessageCount() - $message_count);
               }
               $participant->setParticipationStatus($behind);
               $participant->setDateTouched($time);
             } else {
               $participant->setSeenMessageCount($object->getMessageCount());
               $participant->setBehindTransactionPHID($xaction_phid);
               $participant->setParticipationStatus($up_to_date);
               $participant->setDateTouched($time);
             }
             $participant->save();
           }
 
           PhabricatorUserCache::clearCaches(
             PhabricatorUserMessageCountCacheType::KEY_COUNT,
             array_keys($participants));
 
           break;
       }
     }
 
     if ($xactions) {
       $data = array(
         'type'        => 'message',
         'threadPHID'  => $object->getPHID(),
         'messageID'   => last($xactions)->getID(),
         'subscribers' => array($object->getPHID()),
       );
 
       PhabricatorNotificationClient::tryToPostMessage($data);
     }
 
     return $xactions;
   }
 
   protected function requireCapabilities(
     PhabricatorLiskDAO $object,
     PhabricatorApplicationTransaction $xaction) {
 
     parent::requireCapabilities($object, $xaction);
 
     switch ($xaction->getTransactionType()) {
       case ConpherenceTransaction::TYPE_PARTICIPANTS:
         $old_map = array_fuse($xaction->getOldValue());
         $new_map = array_fuse($xaction->getNewValue());
 
         $add = array_keys(array_diff_key($new_map, $old_map));
         $rem = array_keys(array_diff_key($old_map, $new_map));
 
-        $actor_phid = $this->requireActor()->getPHID();
-
-        // You need CAN_EDIT to change participants other than yourself.
-        PhabricatorPolicyFilter::requireCapability(
-          $this->requireActor(),
-          $object,
-          PhabricatorPolicyCapability::CAN_EDIT);
+        $actor_phid = $this->getActingAsPHID();
+
+        $is_join = (($add === array($actor_phid)) && !$rem);
+        $is_leave = (($rem === array($actor_phid)) && !$add);
+
+        if ($is_join) {
+          // Anyone can join a thread they can see.
+        } else if ($is_leave) {
+          // Anyone can leave a thread.
+        } else {
+          // You need CAN_EDIT to add or remove participants. For additional
+          // discussion, see D17696 and T4411.
+          PhabricatorPolicyFilter::requireCapability(
+            $this->requireActor(),
+            $object,
+            PhabricatorPolicyCapability::CAN_EDIT);
+        }
 
         break;
       case ConpherenceThreadTitleTransaction::TRANSACTIONTYPE:
       case ConpherenceThreadTopicTransaction::TRANSACTIONTYPE:
         PhabricatorPolicyFilter::requireCapability(
           $this->requireActor(),
           $object,
           PhabricatorPolicyCapability::CAN_EDIT);
         break;
     }
   }
 
   protected function mergeTransactions(
     PhabricatorApplicationTransaction $u,
     PhabricatorApplicationTransaction $v) {
 
     $type = $u->getTransactionType();
     switch ($type) {
       case ConpherenceTransaction::TYPE_PARTICIPANTS:
         return $this->mergePHIDOrEdgeTransactions($u, $v);
     }
 
     return parent::mergeTransactions($u, $v);
   }
 
   protected function shouldSendMail(
     PhabricatorLiskDAO $object,
     array $xactions) {
     return true;
   }
 
   protected function buildReplyHandler(PhabricatorLiskDAO $object) {
     return id(new ConpherenceReplyHandler())
       ->setActor($this->getActor())
       ->setMailReceiver($object);
   }
 
   protected function buildMailTemplate(PhabricatorLiskDAO $object) {
     $id = $object->getID();
     $title = $object->getTitle();
     if (!$title) {
       $title = pht(
         '%s sent you a message.',
         $this->getActor()->getUserName());
     }
     $phid = $object->getPHID();
 
     return id(new PhabricatorMetaMTAMail())
       ->setSubject("Z{$id}: {$title}")
       ->addHeader('Thread-Topic', "Z{$id}: {$phid}");
   }
 
   protected function getMailTo(PhabricatorLiskDAO $object) {
     $to_phids = array();
 
     $participants = $object->getParticipants();
     if (!$participants) {
       return $to_phids;
     }
 
     $participant_phids = mpull($participants, 'getParticipantPHID');
 
     $users = id(new PhabricatorPeopleQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withPHIDs($participant_phids)
       ->needUserSettings(true)
       ->execute();
     $users = mpull($users, null, 'getPHID');
 
     $notification_key = PhabricatorConpherenceNotificationsSetting::SETTINGKEY;
     $notification_email =
       PhabricatorConpherenceNotificationsSetting::VALUE_CONPHERENCE_EMAIL;
 
     foreach ($participants as $phid => $participant) {
       $user = idx($users, $phid);
       if ($user) {
         $default = $user->getUserSetting($notification_key);
       } else {
         $default = $notification_email;
       }
 
       $settings = $participant->getSettings();
       $notifications = idx($settings, 'notifications', $default);
 
       if ($notifications == $notification_email) {
         $to_phids[] = $phid;
       }
     }
 
     return $to_phids;
   }
 
   protected function getMailCC(PhabricatorLiskDAO $object) {
     return array();
   }
 
   protected function buildMailBody(
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $body = parent::buildMailBody($object, $xactions);
     $body->addLinkSection(
       pht('CONPHERENCE DETAIL'),
       PhabricatorEnv::getProductionURI('/'.$object->getMonogram()));
 
     return $body;
   }
 
   protected function addEmailPreferenceSectionToMailBody(
     PhabricatorMetaMTAMailBody $body,
     PhabricatorLiskDAO $object,
     array $xactions) {
 
     $href = PhabricatorEnv::getProductionURI(
       '/'.$object->getMonogram().'?settings');
     $label = pht('EMAIL PREFERENCES FOR THIS ROOM');
     $body->addLinkSection($label, $href);
   }
 
   protected function getMailSubjectPrefix() {
     return PhabricatorEnv::getEnvConfig('metamta.conpherence.subject-prefix');
   }
 
   protected function supportsSearch() {
     return true;
   }
 
   protected function validateTransaction(
     PhabricatorLiskDAO $object,
     $type,
     array $xactions) {
 
     $errors = parent::validateTransaction($object, $type, $xactions);
 
     switch ($type) {
       case ConpherenceTransaction::TYPE_PARTICIPANTS:
         foreach ($xactions as $xaction) {
           $new_phids = $this->getPHIDTransactionNewValue($xaction, array());
           $old_phids = nonempty($object->getParticipantPHIDs(), array());
           $phids = array_diff($new_phids, $old_phids);
 
           if (!$phids) {
             continue;
           }
 
           $users = id(new PhabricatorPeopleQuery())
             ->setViewer($this->requireActor())
             ->withPHIDs($phids)
             ->execute();
           $users = mpull($users, null, 'getPHID');
           foreach ($phids as $phid) {
             if (isset($users[$phid])) {
               continue;
             }
             $errors[] = new PhabricatorApplicationTransactionValidationError(
               $type,
               pht('Invalid'),
               pht('New room participant "%s" is not a valid user.', $phid),
               $xaction);
           }
         }
         break;
     }
 
     return $errors;
   }
 }