FrontendUser::isInUserGroup() 

\nn\t3::FrontendUser()->isInUserGroup($feGroups = NULL); 

Checks whether the current frontend user is within a specific user group.

\nn\t3::FrontendUser()->isInUserGroup( 1 );
\nn\t3::FrontendUser()->isInUserGroup( ObjectStorage );
\nn\t3::FrontendUser()->isInUserGroup( [FrontendUserGroup, FrontendUserGroup, ...] );
\nn\t3::FrontendUser()->isInUserGroup( [['uid'=>1, ...], ['uid'=>2, ...]] );
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| @return boolean

Source Code 

public function isInUserGroup( $feGroups = null )
{
	if (!$this->isLoggedIn()) return false;
	$groupsByUid = $this->getCurrentUserGroups();
	$feGroupUids = [];
	if (is_int( $feGroups)) {
		$feGroupUids = [$feGroups];
	} else {
		foreach ($feGroups as $obj) {
			$uid = false;
			if (is_numeric($obj)) $uid = $obj;
			if (is_array($obj) && isset($obj['uid'])) $uid = $obj['uid'];
			if (is_object($obj) && method_exists($obj, 'getUid')) $uid = $obj->getUid();
			if ($uid) $feGroupUids[] = $uid;
		}
	}
	$matches = array_intersect( array_keys($groupsByUid), $feGroupUids );
	return count($matches) > 0;
}
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