ranks (or usergroups) based on profile fields?

[Sculli]

To expand on my mutually exclusive usergroups post. Would it be possible to assign user ranks or non-public usergroups based on profile fields?

For instance, to realize the mutually exclusivness, one could use a radio button user profile, that way only one can be selected.

Would this work? If so, what would be the code to either assign a rank or a non-public user group based on a single-selection radio buttons profile field?