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?

See has_many :through - Self-referential has_many :through associations