[Admin]

In register.php, find these bits of code:

    if ($moderatenewmembers or $coppauser) {
      $newusergroupid=4;
    } else {
      $newusergroupid=[high]2[/high];
    }

			$DB_site->query("UPDATE user SET usergroupid=[high]2[/high] WHERE userid='$u'");

And change the [high]2[/high] in both to the ID of the "Newbies" usergroup.