Send the user ID matching the guest access token, since there is no Matrix API to discover what user ID an access token is for.

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:

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,
             }
         )