My guess is that the filters that are available now are based on metadata that's actually written locally to the files themselves or the local song DB, if you look at the filters that are available now. The availability of the songs online in group play is determined on the fly and is not written to a local DB, and likely remains part of session data. We also don't know if this session data persists or not, or if it is determined upon reentry to the lobby/song selection. It could also be that song caches are shared and compared between users upon session initiation. That being said, however, it still seems that if a) and b) above are true and known, then the system, again, seems like it should have the ability to say "hide" instead of "show but make gray and unselectable" when a song is encountered that is not mutually available.