How To Use Appriateness Of Service Classes

1) I'd like to make use of the built in membership system. Easy enough by itself. However, I wonder if I should still be building an entity around the User and Role classes since they aren't part of the custom database. As such, they aren't part of the domain's aggregate chains either.2) My entities contain the better part of my business logic. For example, my Category entity has the following method: AddForum(User user, Forum forum). All logic tests are performed there and applied, or not, to the private forums collection. What I am wondering about here is the appriateness of service classes. In the traditional mvc style, a service class operates between the controller and repository to perform business logic. With domain design, we do it in the entities. Should I still be using these, now superfluous service classes? Or should I save them for when something is really needed that isn't part of the domain itself, such as an EmailService