Different kinds of Users (roles) should I use inheritance?

Hi, in my application I have 3 different kinds of Users

Patients
Therapists
Admins

For the sake of it, Admins are not important here, what is important is
that a Patient belongs_to a Therapist, and likewise a Therapist has_many
Patients. Good enough so far

I am using restful_authentication FWIW, and have a User model that
people can use to signup and sign in to the application. I have in the
past just used a “role” field to differentiate what people can see, etc.
But I am stuck trying to figure out how to get the relationship between
Therapists and Patients working together. Do I subclass User into 2
models, and then create a join table? Or is there an easier solution?

Thanks for any guidance,
Cheers,
Timothy J.
(w) www.foundinteractive.com