Last month an update to the built-in Chromecast app allowed some Android TV devices to be added to speaker groups before mysteriously disappearing a few days later. Well, after yet another update to this app, the feature is back once again.
For those unfamiliar, speaker groups allow you to group different Cast-enabled devices together. Then when you cast music to that group, all speakers assigned to it will play your music as one.
Some Android TV devices strangely appear to be unsupported at this time, with my Shield TV nowhere to be seen, but my 2016 Phillips TV (49PUS6401/12) showing up as it should be. This was an issue the last time this feature surfaced, so it's not unexpected, but hopefully all Android TV devices will soon be compatible.
According to our tipsters, if you had already added an Android TV device to a group last month before the feature disappeared, then they'll magically pop back up where you left them. If, like me, this is the first time you've attempted to use this, you can manually assign them to a group like any other speaker.
It seems this is linked to both a beta update to the Chromecast app and a server-side flag at the same time. If you're not enrolled in the beta for this app, you can sideload the APK here and see if it works for you.