[TYPO3-mvc] read only for fe_users and usergroup

[Domi]

Hello,

I was trying to set up a validator which is reading out fe_users and its 
usergroup(s).

$query = $this->frontendUserRepository->createQuery();
$query->getQuerySettings()->setRespectStoragePage(false);
$query->matching($query->equals('username', $username));
$user = $query->execute()->getFirst();

$userGroup = $user->getUsergroup()->toArray();

But $userGroup remains empty. What typoscript settings do I need? I want 
a simple readOut, not data manipulation.

Thanks,

Dominic