Press and hold the Play/Pause button on a player to group it with the last active room.
This makes up for not being able to group rooms via Alexa.(Although I still hope it's added) If I walk into a room that isn't part of my group I just hold the Play/Pause button down and its added to the group. Don't have to find my phone or open the App. Works great!
I use it a lot if I'm watching TV and walk into the kitchen to do something. Press the button and you're grouped.