I always wondered why this wasn't part of Chromecasts already. This has been possible with Chromecast Audio devices for a couple years now, but I've never been able to add the Chromecast on my TV to the group.
Chromecast is still a little more wonky than the competition, but well worth the very small investment required. Last year we through a huge end of summer BBQ, and I decided to spend $70 on a couple more Chromecast audios. I put one on a set of computer speakers on the back deck, and another in our front room where the appetizer table was, and we filled the party area with synchronized music for approximately $1000 less than a Sonos system. Sure, it didn't sound as nice, but when everyone is talking, who cares?