All it does is allow you to combine multiple users into one "show posts by user" query. Hit up the profile page of the person you wish to add/remove from your buddy list and look at the bottom for the link.
As soon as you have a non-empty buddy list, a "Show posts by W's in your buddy list" link should be added into the control bar of your index.
This may or may not work and should be considered BETA for now. Please report any problems in a reply to this thread, or drop me a private message.
Later, I may make it easier to add/remove buddies without visiting a profile page first. I may also add cute graphics. I may not! I'm unpredictable!
Is there going to be any way to see who a user has on their buddy list, or who has them on theirs (kind of a friend/friend of type thing)? I think it'd be fun to find out who the stalkers are.