This is really nice, congrats!

Since it's YouTube would you consider chromecast support from the host machine? I'm honestly not sure exactly how it works but it could be neat to have the videos on the TV and that's quite often where the best sound system is.

Also have you considered letting people set the party url? Taking advantage of the fact most could probably come up with a reasonably unique and memorable name and the fact you only need to keep them around for a few hrs or a day - it could make the connection easier.