Do you remember Turntable.fm before they gave up on the group listening thing? QCast is the same idea, but it sends tunes from your phone to a Chromecast and anyone can contribute a track. Unlike Turntable, QCast isn't handling any of the music licensing. It just plugs into Google Play Music All Access. It's also for real life gatherings, not random people on the internet.
Every party needs a host, and that person must have an All Access subscription.