Hi Alan
Try this code below (replace 100, 101 with IDs of the user groups you want)
Code:
$user = JFactory::getUser($row->user_id);
$currentGroups = $user->get('groups');
$newGroups = [100, 101];
$currentGroups = array_unique(array_merge($currentGroups, $newGroups));
$user->set('groups', $currentGroups);
$user->save(true);
Then check to see whether it works
Tuan
