[wrestlepower]

may not quite what you are hoping to achieve, but you could always edit the options for the usergroups (light and dark) to make them joinable by members. That way they would be able to selct which usergroup to join through their usercp in the miscellaneous section under group memberships.

If you want it to be more like you mentioned in your post, it should be possible to create the profile field and then edit some of the templates to achieve the required effect but that would be a lot more work.