use $this->userRepository from groupaction

src/Application/Actions/Group/AddUserGroupAction.php

@@ -20,8 +20,7 @@ class AddUserGroupAction extends GroupAction
         if (!isset($group)) {
             throw new GroupNotFoundException();
         }
-        $userRepository = $this->em->getRepository('App\Domain\User\User');
-        $currentUser = $userRepository->find($_SESSION['userId']);
+        $currentUser = $this->userRepository->find($_SESSION['userId']);
         $currentUser->addGroup($group);        
 
         $this->em->persist($group);