Checking user in group

Would you so kind to help me with my problem concerning Nextcloud.
I have  Nextcloud 14.0.3 and  operating system is Ubuntu 18.04.1 LTS.

By using Owncloud:
 Im getting user name
    $UserName= \OC_User::getUser();
 Checking user in group TestGrouapName
    if( OC_Group::inGroup( OC_User::getUser(), 'TestGroupName' ) === false ){
               //...
    }

By using Nextcloud:
 Im getting user name
    $UserName= \OC_User::getUser(); //Its ok
 Donit work Checking user in group TestGrouapName
    OC_Group::inGroup( OC_User::getUser(), 'TestGroupName' )

How can I check the user in group?

Do you have any ideas?...

Best regards.