add user repository in construct

src/Application/Actions/Group/GroupAction.php

@@ -25,6 +25,7 @@ abstract class GroupAction extends Action
         parent::__construct($logger);
         $this->em = $em;
         $this->groupRepository = $em->getRepository('App\Domain\Group\Group');
+        $this->userRepository = $em->getRepository('App\Domain\User\User');
         $this->twig = $twig;
         $this->flash = $flash;
     }