Another possibility is BandHub. This probably isn't quite what you're looking for, but it's essentially a multi-track youtube, with various people able to record & upload new video tracks to a song. It's focused more on recording, than on composing, arranging, mixing, or mastering.
My experience is that it's not so good with the post-production & editing side of things. For example, AFAICT, it doesn't really give you a chance to edit what you record before uploading; once you've recorded a track, you either upload it right then, or you don't. Of course, it's been a while since I've looked at it, so things may have changed.