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
@@ -2550,6 +2550,7 @@
     'PhabricatorSubscriptionsEditController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsEditController.php',
     'PhabricatorSubscriptionsEditor' => 'applications/subscriptions/editor/PhabricatorSubscriptionsEditor.php',
     'PhabricatorSubscriptionsListController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsListController.php',
+    'PhabricatorSubscriptionsSubscribeEmailCommand' => 'applications/subscriptions/command/PhabricatorSubscriptionsSubscribeEmailCommand.php',
     'PhabricatorSubscriptionsTransactionController' => 'applications/subscriptions/controller/PhabricatorSubscriptionsTransactionController.php',
     'PhabricatorSubscriptionsUIEventListener' => 'applications/subscriptions/events/PhabricatorSubscriptionsUIEventListener.php',
     'PhabricatorSubscriptionsUnsubscribeEmailCommand' => 'applications/subscriptions/command/PhabricatorSubscriptionsUnsubscribeEmailCommand.php',
@@ -5937,6 +5938,7 @@
     'PhabricatorSubscriptionsEditController' => 'PhabricatorController',
     'PhabricatorSubscriptionsEditor' => 'PhabricatorEditor',
     'PhabricatorSubscriptionsListController' => 'PhabricatorController',
+    'PhabricatorSubscriptionsSubscribeEmailCommand' => 'MetaMTAEmailTransactionCommand',
     'PhabricatorSubscriptionsTransactionController' => 'PhabricatorController',
     'PhabricatorSubscriptionsUIEventListener' => 'PhabricatorEventListener',
     'PhabricatorSubscriptionsUnsubscribeEmailCommand' => 'MetaMTAEmailTransactionCommand',
diff --git a/src/applications/subscriptions/command/PhabricatorSubscriptionsSubscribeEmailCommand.php b/src/applications/subscriptions/command/PhabricatorSubscriptionsSubscribeEmailCommand.php
new file mode 100644
--- /dev/null
+++ b/src/applications/subscriptions/command/PhabricatorSubscriptionsSubscribeEmailCommand.php
@@ -0,0 +1,74 @@
+<?php
+
+final class PhabricatorSubscriptionsSubscribeEmailCommand
+  extends MetaMTAEmailTransactionCommand {
+
+  public function getCommand() {
+    return 'subscribe';
+  }
+
+  public function getCommandSyntax() {
+    return '**!subscribe** //username #project ...//';
+  }
+
+  public function getCommandSummary() {
+    return pht('Add users or projects as subscribers.');
+  }
+
+  public function getCommandDescription() {
+    return pht(
+      'Add one or more subscribers to the object. You can add users '.
+      'by providing their usernames, or add projects by adding their '.
+      'hashtags. For example, use `!subscribe alincoln #ios` to add the '.
+      'user `alincoln` and the project with hashtag `#ios` as subscribers.'.
+      "\n\n".
+      'Subscribers which are invalid or unrecognized will be ignored. This '.
+      'command has no effect if you do not specify any subscribers.'.
+      "\n\n".
+      'Users who are CC\'d on the email itself are also automatically '.
+      'subscribed if Phabricator knows which accounts are linked to their '.
+      'email addresses.');
+  }
+
+  public function getCommandAliases() {
+    return array(
+      'cc',
+    );
+  }
+
+  public function isCommandSupportedForObject(
+    PhabricatorApplicationTransactionInterface $object) {
+    return ($object instanceof PhabricatorSubscribableInterface);
+  }
+
+  public function buildTransactions(
+    PhabricatorUser $viewer,
+    PhabricatorApplicationTransactionInterface $object,
+    PhabricatorMetaMTAReceivedMail $mail,
+    $command,
+    array $argv) {
+
+    $subscriber_phids = id(new PhabricatorObjectListQuery())
+      ->setViewer($viewer)
+      ->setAllowedTypes(
+        array(
+          PhabricatorPeopleUserPHIDType::TYPECONST,
+          PhabricatorProjectProjectPHIDType::TYPECONST,
+        ))
+      ->setObjectList(implode(' ', $argv))
+      ->setAllowPartialResults(true)
+      ->execute();
+
+    $xactions = array();
+
+    $xactions[] = $object->getApplicationTransactionTemplate()
+      ->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS)
+      ->setNewValue(
+        array(
+          '+' => array_fuse($subscriber_phids),
+        ));
+
+    return $xactions;
+  }
+
+}