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.

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).

J1.5 doesn't support custom usergroups nor does CB support said custom usergroups on J1.5. I suggest upgrading to J1.7. I can't suggest an ACL query on J1.5 that is not supported, sorry. You'll need to review database and construct one your self.

GroupJive has nothing to do with ACL so no it could not help in that regard. GroupJive provides groups as in how Facebook, MySpace, or any other social site has groups. You can of course alter a users GJ attendance (create category, create group, join group) automatically for example using the Auto integration included with the GJ package. It can also be done through CBSubs GJ integration or through incubator Auto Actions project.