Using Models as Join Tables and/or Self Referential Joins?

Given a User model, I would like to describe a relationship between
two users and add additional information about the connection. For
example, user A and B msg'd each other on X date, and are Y miles
apart. In the book Agile Web Dev with Rails (pg 337), they describe
using models as join tables to add more information than foreign
keys. However, join table models seems to only help describe
relationships if you have two models.

What do you use if you only have one User model and want to add
additional information about relationships between users?