This has gone a bit down a different path, I'm familiar with the setup of multiroom groups and have a 'Downstairs' group setup with two devices.
What I'm interested in is whether you can have one device 'join in' if another is already playing, at the moment the only way to get a multiroom group to play is to start playing something new.
I'd see the flow to be something like..
Echo A playing from Amazon Music and is in the middle of a song
Echo B is asked to start playing as well - this activates the multiroom group and Echo B starts playing the same song/playlist that Echo A is playing
This might not be possible yet! was just interested if anyone knew if it did.