AcmeDemoBundle::UserAdminController.php

public function batchActionAddToCollection(ProxyQueryInterface $selectedModelQuery)
    {

        $request = $this->get('request');

        $group = $this
            ->getDoctrine()
            ->getRepository('VelvelZDFBundle:Group')
            ->find($request->request->get('secondary'));

        $users = $selectedModelQuery->execute();

        foreach ($users as $user)
        {
            $group->addUser($user);
        }

        $this
            ->getDoctrine()
            ->getManager()
            ->flush();

        $this
            ->get('session')
            ->setFlash('sonata_flash_success', 'flash_batch_merge_success');

        return new RedirectResponse($this->admin->generateUrl('list', $this->admin->getFilterParameters()));
    }