Any buddypress experts here? Question!

Hey guys, I am creating an “e-mentoring” website where mentors and mentees can sign up and provide advice and discussion to each other. However, when a mentor registers, I want them to be able to click a drop down box that will say “Mentor” or “Mentee”, and when they choose “Mentor”, I want WP to assign them the role of “Author”. If a mentee registeres, he can choose “Mentee” and then WP will auto assign him the role of “subscriber”.

How do I do this? Is there a plugin I can use? Thanks!