HOW DO I automatically add someone to a Usergroup based on a Profile field?

[ataraxia]

HOW DO I automatically add someone to a Usergroup based on a Profile field?

For example, if I have a single choice Profile field for, say, "Gender" with possible responses of "Male", "Female" and "Other", is there some way they could automatically be added as an "additional" member to my corresponding "male", "female" or "other" usergroup?

(A 2nd, related question is how do I update my existing users based in this?)

Thanks