FrontendUser::resolveUserGroups() 

\nn\t3::FrontendUser()->resolveUserGroups($arr = [], $ignoreUids = []); 

Converts an array or a comma-separated list with user group UIDs into | fe_user_groups data from the database. Checks for inherited subgroup.

\nn\t3::FrontendUser()->resolveUserGroups( [1,2,3] );
\nn\t3::FrontendUser()->resolveUserGroups( '1,2,3' );
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| @return array

Source Code 

public function resolveUserGroups( $arr = [], $ignoreUids = [] )
{
	$arr = \nn\t3::Arrays( $arr )->intExplode();
	if (!$arr) return [];
	return GeneralUtility::makeInstance(\TYPO3\CMS\Core\Authentication\GroupResolver::class)->resolveGroupsForUser(['usergroup'=>join(',', $arr)], 'fe_groups');
}
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