My team and I built QCast at HackSC and won second place! QCast turns your Chromecast into a shared party playlist.
When you decide to host a party, your house guests will not need to sign up for any accounts to contribute to the playlist. With just one Google Play All Access account, everybody can add any song they can think of to it.
We used an Unofficial Google Play Music API with some modifications of our own to make it possible. All playlist management is done on a $35 Chromecast with no other setup necessary. Any guest with an Android phone will be able to join.