I can't seem to even find a support page relevant to this...
I was at work and my roommate wanted to use the Nexus Player(which is under my account). He first played music directly from the Play Music app, but then realized once it stopped him that I was using it too(Only one device per account). So he switched to streaming from his account, but at some point it switched back over to using my account, which paused my music at work again.
We really need multi-user support on Android TV. We keep separate accounts so we have separate music libraries, and being able to access each of our youtube accounts would be great too.
I agree, that's why I can't dump my Chromecast yet. The native apps are a nice thing to have but googlecast should behave exactly like a Chromecast. You can achieve what you want with a Chromecast and casting.