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. Once the host is logged into Google in QCast, everyone else can connect to the Chromecast and piggyback on the one subscription. From there you simply search for tracks and add them. The host retains the ability to remove songs from the queue and control the volume, though.
Everyone has the option to downvote songs they don't like, and if a majority agree, the song is automatically skipped. Again, that's pretty much how Turntable worked. The app seems to work as advertised, but the search is a little wonky right now. I'm not seeing all the tracks I'd expect when doing some sample searches. Still, it's free and could be really neat if you have an All Access subscription anyway.