A user would have a preference variable saying, essentially "let others use my friends groups". Right now, if I'm logged in as first_user, and am trying to view http://www.livejournal.com/users/second_user/some_group , what I get is the complete friends view of second_user, not restricted to "some_group". This makes sense, because if I could see just the posts of users from "some_group", I could know who belongs to "some_group" and who doesn't, but this information should be private to second_user.
However, maybe second_user doesn't really care about that, and is willing to let me and everyone else see his friends views restricted to "some_group". Maybe he's got some interesting subset of users in "some_group" that is fascinating to watch. Right now it can't be done. But we could have a preference variable for second_user saying "let others watch my groups-restricted views".
This is not hard to implement. I wonder, however, whether many people would find it a useful feature.