CB Fields can't do that, you would need CB Queries. ACL is very complex, changing it via a query is a terrible idea. Especially in J1.7 where it is heavily cached into API and Session so changing it via query would have zero immediate effect for the user. At any rate the below will do it (J1.7, you provided no version), test extremely thoroughly.
Code:
UPDATE `#__users` SET `usertype` = 'Girls' WHERE `id` = [user_id]
REPLACE INTO `#__user_usergroup_map` ( `user_id`, `group_id` ) VALUES ( [user_id], 33 )
DELETE FROM `#__user_usergroup_map` WHERE `user_id` = [user_id] AND `group_id` != 33
With the above it'll set the usertype, add or replace (if it already exists) the girls usergroup, then remove all usergroups except the girls usergroup (optional on J1.7 if you want multi-usergroups).