diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -1498,6 +1498,7 @@
     'PhabricatorCalendarEventCommentController' => 'applications/calendar/controller/PhabricatorCalendarEventCommentController.php',
     'PhabricatorCalendarEventEditController' => 'applications/calendar/controller/PhabricatorCalendarEventEditController.php',
     'PhabricatorCalendarEventEditor' => 'applications/calendar/editor/PhabricatorCalendarEventEditor.php',
+    'PhabricatorCalendarEventEmailCommand' => 'applications/calendar/command/PhabricatorCalendarEventEmailCommand.php',
     'PhabricatorCalendarEventInvalidEpochException' => 'applications/calendar/exception/PhabricatorCalendarEventInvalidEpochException.php',
     'PhabricatorCalendarEventInvitee' => 'applications/calendar/storage/PhabricatorCalendarEventInvitee.php',
     'PhabricatorCalendarEventInviteeQuery' => 'applications/calendar/query/PhabricatorCalendarEventInviteeQuery.php',
@@ -1506,6 +1507,7 @@
     'PhabricatorCalendarEventMailReceiver' => 'applications/calendar/mail/PhabricatorCalendarEventMailReceiver.php',
     'PhabricatorCalendarEventPHIDType' => 'applications/calendar/phid/PhabricatorCalendarEventPHIDType.php',
     'PhabricatorCalendarEventQuery' => 'applications/calendar/query/PhabricatorCalendarEventQuery.php',
+    'PhabricatorCalendarEventRSVPEmailCommand' => 'applications/calendar/command/PhabricatorCalendarEventRSVPEmailCommand.php',
     'PhabricatorCalendarEventSearchEngine' => 'applications/calendar/query/PhabricatorCalendarEventSearchEngine.php',
     'PhabricatorCalendarEventSearchIndexer' => 'applications/calendar/search/PhabricatorCalendarEventSearchIndexer.php',
     'PhabricatorCalendarEventTransaction' => 'applications/calendar/storage/PhabricatorCalendarEventTransaction.php',
@@ -4848,6 +4850,7 @@
     'PhabricatorCalendarEventCommentController' => 'PhabricatorCalendarController',
     'PhabricatorCalendarEventEditController' => 'PhabricatorCalendarController',
     'PhabricatorCalendarEventEditor' => 'PhabricatorApplicationTransactionEditor',
+    'PhabricatorCalendarEventEmailCommand' => 'MetaMTAEmailTransactionCommand',
     'PhabricatorCalendarEventInvalidEpochException' => 'Exception',
     'PhabricatorCalendarEventInvitee' => array(
       'PhabricatorCalendarDAO',
@@ -4859,6 +4862,7 @@
     'PhabricatorCalendarEventMailReceiver' => 'PhabricatorObjectMailReceiver',
     'PhabricatorCalendarEventPHIDType' => 'PhabricatorPHIDType',
     'PhabricatorCalendarEventQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
+    'PhabricatorCalendarEventRSVPEmailCommand' => 'PhabricatorCalendarEventEmailCommand',
     'PhabricatorCalendarEventSearchEngine' => 'PhabricatorApplicationSearchEngine',
     'PhabricatorCalendarEventSearchIndexer' => 'PhabricatorSearchDocumentIndexer',
     'PhabricatorCalendarEventTransaction' => 'PhabricatorApplicationTransaction',
diff --git a/src/applications/calendar/command/PhabricatorCalendarEventEmailCommand.php b/src/applications/calendar/command/PhabricatorCalendarEventEmailCommand.php
new file mode 100644
--- /dev/null
+++ b/src/applications/calendar/command/PhabricatorCalendarEventEmailCommand.php
@@ -0,0 +1,11 @@
+<?php
+
+abstract class PhabricatorCalendarEventEmailCommand
+  extends MetaMTAEmailTransactionCommand {
+
+  public function isCommandSupportedForObject(
+    PhabricatorApplicationTransactionInterface $object) {
+    return ($object instanceof PhabricatorCalendarEvent);
+  }
+
+}
diff --git a/src/applications/calendar/command/PhabricatorCalendarEventRSVPEmailCommand.php b/src/applications/calendar/command/PhabricatorCalendarEventRSVPEmailCommand.php
new file mode 100644
--- /dev/null
+++ b/src/applications/calendar/command/PhabricatorCalendarEventRSVPEmailCommand.php
@@ -0,0 +1,124 @@
+<?php
+
+final class PhabricatorCalendarEventRSVPEmailCommand
+  extends PhabricatorCalendarEventEmailCommand {
+
+  public function getCommand() {
+    return 'rsvp';
+  }
+
+  public function getCommandSyntax() {
+    return '**!rsvp** //rsvp//';
+  }
+
+  public function getCommandSummary() {
+    return pht('RSVP to event.');
+  }
+
+  public function getCommandDescription() {
+    $status_attending = PhabricatorCalendarEventInvitee::STATUS_ATTENDING;
+    $status_declined = PhabricatorCalendarEventInvitee::STATUS_DECLINED;
+
+    $yes_values = implode(', ', $this->getYesValues());
+    $no_values = implode(', ', $this->getNoValues());
+
+    $table = array();
+    $table[] = '| '.pht('RSVP').' | '.pht('Keywords');
+    $table[] = '|---|---|';
+    $table[] = '| '.$status_attending.' | '.$yes_values;
+    $table[] = '| '.$status_declined.' | '.$no_values;
+    $table = implode("\n", $table);
+
+    return pht(
+      'To RSVP to the event, specify the desired RSVP, like '.
+      '`!rsvp yes`. This table shows the configured names for rsvp\'s.'.
+      "\n\n%s\n\n".
+      'If you specify an invalid rsvp, the command is ignored. This '.
+      'command has no effect if you do not specify an rsvp.',
+      $table);
+  }
+
+  public function buildTransactions(
+    PhabricatorUser $viewer,
+    PhabricatorApplicationTransactionInterface $object,
+    PhabricatorMetaMTAReceivedMail $mail,
+    $command,
+    array $argv) {
+    $status_attending = PhabricatorCalendarEventInvitee::STATUS_ATTENDING;
+    $status_declined = PhabricatorCalendarEventInvitee::STATUS_DECLINED;
+    $xactions = array();
+
+    $target = phutil_utf8_strtolower(implode(' ', $argv));
+    $rsvp = null;
+
+    $yes_values = $this->getYesValues();
+    $no_values = $this->getNoValues();
+
+    if (in_array($target, $yes_values)) {
+      $rsvp = $status_attending;
+    } else if (in_array($target, $no_values)) {
+      $rsvp = $status_declined;
+    } else {
+      $rsvp = null;
+    }
+
+    if ($rsvp === null) {
+      return array();
+    }
+
+    $xactions[] = $object->getApplicationTransactionTemplate()
+      ->setTransactionType(PhabricatorCalendarEventTransaction::TYPE_INVITE)
+      ->setNewValue(array($viewer->getPHID() => $rsvp));
+
+    return $xactions;
+  }
+
+  private function getYesValues() {
+    return array(
+      'yes',
+      'yep',
+      'sounds good',
+      'going',
+      'attending',
+      'will be there',
+      'sure',
+      'accept',
+      'ya',
+      'yeah',
+      'yuh',
+      'uhuh',
+      'ok',
+      'okay',
+      'yiss',
+      'aww yiss',
+      'attend',
+      'intend to attend',
+      'confirm',
+      'confirmed',
+      'bringing dessert',
+      'bringing desert',
+      'time2business',
+      'time4business',
+      );
+  }
+
+  private function getNoValues() {
+    return array(
+      'no',
+      'nope',
+      'no thank you',
+      'next time',
+      'nah',
+      'nuh',
+      'huh',
+      'wut',
+      'no way',
+      'nuhuh',
+      'decline',
+      'declined',
+      'leave',
+      'cancel',
+      );
+  }
+
+}
diff --git a/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php b/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
--- a/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
+++ b/src/applications/calendar/editor/PhabricatorCalendarEventEditor.php
@@ -147,16 +147,8 @@
       case PhabricatorCalendarEventTransaction::TYPE_INVITE:
         $map = $xaction->getNewValue();
         $phids = array_keys($map);
-        $invitees = array();
-
-        if ($map) {
-          $invitees = id(new PhabricatorCalendarEventInviteeQuery())
-            ->setViewer($this->getActor())
-            ->withEventPHIDs(array($object->getPHID()))
-            ->withInviteePHIDs($phids)
-            ->execute();
-          $invitees = mpull($invitees, null, 'getInviteePHID');
-        }
+        $invitees = $object->getInvitees();
+        $invitees = mpull($invitees, null, 'getInviteePHID');
 
         foreach ($phids as $phid) {
           $invitee = idx($invitees, $phid);
@@ -165,10 +157,12 @@
               ->setEventPHID($object->getPHID())
               ->setInviteePHID($phid)
               ->setInviterPHID($this->getActingAsPHID());
+            $invitees[] = $invitee;
           }
           $invitee->setStatus($map[$phid])
             ->save();
         }
+        $object->attachInvitees($invitees);
         return;
       case PhabricatorTransactions::TYPE_COMMENT:
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
@@ -254,14 +248,16 @@
     $phids[] = $this->getActingAsPHID();
 
     $invitees = $object->getInvitees();
-    foreach ($invitees as $phid => $status) {
+    foreach ($invitees as $invitee) {
+      $status = $invitee->getStatus();
       if ($status === PhabricatorCalendarEventInvitee::STATUS_ATTENDING
         || $status === PhabricatorCalendarEventInvitee::STATUS_INVITED) {
-        $phids[] = $phid;
+        $phids[] = $invitee->getInviteePHID();
       }
     }
 
     $phids = array_unique($phids);
+
     return $phids;
   }
 
diff --git a/src/applications/calendar/storage/PhabricatorCalendarEvent.php b/src/applications/calendar/storage/PhabricatorCalendarEvent.php
--- a/src/applications/calendar/storage/PhabricatorCalendarEvent.php
+++ b/src/applications/calendar/storage/PhabricatorCalendarEvent.php
@@ -42,7 +42,7 @@
   }
 
   public function save() {
-    if ($this->getDateTo() <= $this->getDateFrom()) {
+    if ($this->getDateTo() < $this->getDateFrom()) {
       throw new PhabricatorCalendarEventInvalidEpochException();
     }
 
diff --git a/src/applications/calendar/storage/PhabricatorCalendarEventTransaction.php b/src/applications/calendar/storage/PhabricatorCalendarEventTransaction.php
--- a/src/applications/calendar/storage/PhabricatorCalendarEventTransaction.php
+++ b/src/applications/calendar/storage/PhabricatorCalendarEventTransaction.php
@@ -11,7 +11,6 @@
   const TYPE_CANCEL = 'calendar.cancel';
   const TYPE_INVITE = 'calendar.invite';
 
-
   const MAILTAG_RESCHEDULE = 'calendar-reschedule';
   const MAILTAG_CONTENT = 'calendar-content';
   const MAILTAG_OTHER = 'calendar-other';