Sonos "Join" Feature

Where this came from: this question was asked on c4forums/Question & Answer — not here. We answered it there, in public, and this is that answer.

Short answer: there's no long-press Play in Composer. That gesture belongs to Sonos, and Sonos documents it: press and hold play/pause on a product to group it with a room that's playing, and keep holding to cycle through the rooms if several are playing. Nothing in the Control4 Sonos driver's connections passes it to Control4 as an event.
https://support.sonos.com/en-us/article/use-button-controls-on-sonos-products

What Control4 does document is the keypad version of the result. In Composer, program the button with the source room's Digital Media command "Add Rooms" and tick the room that should join. Control4's help walks through it on a 6-button keypad:
https://docs.control4.com/help/c4/software/cpro/dealer-composer-help/content/composerpro_userguide/program_a_button_to_add_a.htm

That gives you "join the kitchen", one room at a time. For "join whatever's playing anywhere" you'd put conditionals on the button that check rooms in priority order and add to the first one that's on. It works, but it gets long, so I'd only do it for the two or three rooms that actually get followed.

One snag owners have reported on Sonos's community: grouping from Control4 works while listening to music, but not when a room is in a Watch session (a TV feeding an Arc, for example).
https://en.community.sonos.com/smart-home-integrations-229108/control4-unable-to-group-zones-when-using-the-watch-activity-6929903

I'd stay away from sending Sonos's local UPnP commands from a generic driver. Sonos doesn't document that interface publicly (its public Control API is cloud-only), so a command that works today can break on the next Sonos update, and Control4 won't know the group changed.

If you post your OS version and the Sonos driver version from Composer, I can tell you what that version of the driver exposes. It's one button's worth of programming.

For transparency: I'm a Control4 and Lutron dealer and do remote programming work.