diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 183245443..3038df4ab 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -534,7 +534,7 @@ class Auth(object):
                 )
 
             access_token = request.args["access_token"][0]
-            user_info = yield self._get_user_by_access_token(access_token)
+            user_info = yield self.get_user_by_access_token(access_token)
             user = user_info["user"]
             token_id = user_info["token_id"]
             is_guest = user_info["is_guest"]
@@ -595,7 +595,7 @@ class Auth(object):
         defer.returnValue(user_id)
 
     @defer.inlineCallbacks
-    def _get_user_by_access_token(self, token):
+    def get_user_by_access_token(self, token):
         """ Get a registered user's ID.
 
         Args:
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 0cb6c521c..57113ae4a 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -884,6 +884,10 @@ class RoomMemberHandler(BaseHandler):
             inviter_user_id=inviter_user_id,
         )
 
+        guest_user_info = yield self.hs.get_auth().get_user_by_access_token(
+            guest_access_token
+        )
+
         is_url = "%s%s/_matrix/identity/api/v1/store-invite" % (
             id_server_scheme, id_server,
         )
@@ -900,6 +904,7 @@ class RoomMemberHandler(BaseHandler):
                 "sender": inviter_user_id,
                 "sender_display_name": inviter_display_name,
                 "sender_avatar_url": inviter_avatar_url,
+                "guest_user_id": guest_user_info["user"].to_string(),
                 "guest_access_token": guest_access_token,
             }
         )
diff --git a/tests/rest/client/v1/test_rooms.py b/tests/rest/client/v1/test_rooms.py
index fe4fd7127..4ab8b35e6 100644
--- a/tests/rest/client/v1/test_rooms.py
+++ b/tests/rest/client/v1/test_rooms.py
@@ -54,13 +54,13 @@ class RoomPermissionsTestCase(RestTestCase):
 
         hs.get_handlers().federation_handler = Mock()
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
@@ -419,13 +419,13 @@ class RoomsMemberListTestCase(RestTestCase):
 
         self.auth_user_id = self.user_id
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
@@ -501,13 +501,13 @@ class RoomsCreateTestCase(RestTestCase):
 
         hs.get_handlers().federation_handler = Mock()
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
@@ -593,14 +593,14 @@ class RoomTopicTestCase(RestTestCase):
 
         hs.get_handlers().federation_handler = Mock()
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
 
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
@@ -708,13 +708,13 @@ class RoomMemberStateTestCase(RestTestCase):
 
         hs.get_handlers().federation_handler = Mock()
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
@@ -840,13 +840,13 @@ class RoomMessagesTestCase(RestTestCase):
 
         hs.get_handlers().federation_handler = Mock()
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
@@ -942,13 +942,13 @@ class RoomInitialSyncTestCase(RestTestCase):
 
         hs.get_handlers().federation_handler = Mock()
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
@@ -1014,13 +1014,13 @@ class RoomMessageListTestCase(RestTestCase):
 
         hs.get_handlers().federation_handler = Mock()
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
diff --git a/tests/rest/client/v1/test_typing.py b/tests/rest/client/v1/test_typing.py
index 16d788ff6..d0037a53e 100644
--- a/tests/rest/client/v1/test_typing.py
+++ b/tests/rest/client/v1/test_typing.py
@@ -61,14 +61,14 @@ class RoomTypingTestCase(RestTestCase):
 
         hs.get_handlers().federation_handler = Mock()
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.auth_user_id),
                 "token_id": 1,
                 "is_guest": False,
             }
 
-        hs.get_v1auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_v1auth().get_user_by_access_token = get_user_by_access_token
 
         def _insert_client_ip(*args, **kwargs):
             return defer.succeed(None)
diff --git a/tests/rest/client/v2_alpha/__init__.py b/tests/rest/client/v2_alpha/__init__.py
index 84334dce3..5170217d9 100644
--- a/tests/rest/client/v2_alpha/__init__.py
+++ b/tests/rest/client/v2_alpha/__init__.py
@@ -43,13 +43,13 @@ class V2AlphaRestTestCase(unittest.TestCase):
             resource_for_federation=self.mock_resource,
         )
 
-        def _get_user_by_access_token(token=None, allow_guest=False):
+        def get_user_by_access_token(token=None, allow_guest=False):
             return {
                 "user": UserID.from_string(self.USER_ID),
                 "token_id": 1,
                 "is_guest": False,
             }
-        hs.get_auth()._get_user_by_access_token = _get_user_by_access_token
+        hs.get_auth().get_user_by_access_token = get_user_by_access_token
 
         for r in self.TO_REGISTER:
             r.register_servlets(hs, self.mock_resource)