From what I recall, groups in Sonos are more ephemeral in nature (eg. they can be created and destroyed on the fly and don’t exist once the speakers have been ungrouped)… so I don’t think groups get treated as devices in the SmartThings ecosystem.
From a quick search, it sounds like you can’t modify groups from within the SmartThings ecosystem, but if you play music to a speaker that is already part of a group, it will play for the whole group.
Edit: Looks like some community members use a project called Jishi Node Sonos for controlling grouping. It’s a bit more of a technical setup though.
That being said, I’ll leave it to other community members who actively use Sonos groups to comment… otherwise you might try posting in the SmartThings community.