Have Spotify Premium account and have authorised it on the Alexa App.
I have 3 echo devices all setup in one multiroom group.
I can choose and play Spotify on any of my devices individually but Multiroom doesnt appear to be an option.
Are you trying to play music through the Spotify app or are you asking Alexa to play music on Spotify using your multi-room group? If you're doing the former, try the latter. Multi-room groups won't be visible in apps other than the Alexa app.