[TYPO3-mvc] addUsergroup in extended feUser

[Domi]

Hello List,

I struggling with adding usergroups to the extended fe_users table. I 
tried this:

public function createAction(Tx_Intranet_Domain_Model_Client $newClient)
{

$feGroups = $this->frontendUserGroupRepository;
$group = $feGroups->findByUid('4');

$newClient->addUsergroup($group);
$this->clientRepository->add($newClient);

$this->flashMessageContainer->add('Your new Client was created.');

$this->redirect('afterSubmit');
}

How do I rightly add / remove Relations from my domain model? There is 
no error given, it simply results into a white page, the line:
$newClient->addUsergroup($group);
breaks my code.

Thanks!

Dominic