AdminCP > Usergroups > Usergroup Manager > Edit Usergroup
and/or
AdminCP > Users > Search for Users > then search for those users and change their usergroups
Hope that helps
--------------- Added [DATE]1300242400[/DATE] at [TIME]1300242400[/TIME] ---------------
It would also be wise to check your Usergroup Promotions as well, along with any similar modifications you may have installed that could've affected this