diff --git a/src/applications/conpherence/controller/ConpherenceViewController.php b/src/applications/conpherence/controller/ConpherenceViewController.php
--- a/src/applications/conpherence/controller/ConpherenceViewController.php
+++ b/src/applications/conpherence/controller/ConpherenceViewController.php
@@ -147,9 +147,6 @@
     $user = $this->getRequest()->getUser();
 
     $participating = $conpherence->getParticipantIfExists($user->getPHID());
-    if (!$participating && $user->isLoggedIn()) {
-      return null;
-    }
     $draft = PhabricatorDraft::newFromUserAndKey(
       $user,
       $conpherence->getPHID());
diff --git a/src/applications/conpherence/editor/ConpherenceEditor.php b/src/applications/conpherence/editor/ConpherenceEditor.php
--- a/src/applications/conpherence/editor/ConpherenceEditor.php
+++ b/src/applications/conpherence/editor/ConpherenceEditor.php
@@ -341,13 +341,23 @@
         $add = array_keys(array_diff_key($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: