It's covered in the DMG and even moreso in the Epic book.
Followers are from the following classes: Commoner, Warrior or Expert.
In the Epic Leadership feat they include optional rules for allowing Adepts and Aristocrats, but at (+1 or +2, I don't have the book on-hand) levels. They also allow core classes, but at an even greater penalty, and Prestige Classes at +5 levels. (Thus a Rogue 5 / Assassin 1 would be considered a level 11 follower).