I want to be able to programmatically add a user to a secondary group through a PHP script once they've upgraded to premium on my website. I'm looking in Xenforo/Model/UserGroup.php for a function to add a secondary group to a user, but I can't seem to find one. I can find changePrimaryUserGroupForUsers() and removeSecondaryGroupFromUsers() but there doesn't seem to be a function to add secondary groups. Any help would be appreciated. Thanks!