There isn’t.  Alexa grouping cannot group a TV source (or any other “Group on the fly” ad-hoc type grouping).  All Alexa grouping is by initiating playback in a defined group. You can do it via Sonos Voice Assistant.

I wouldn’t have thought that would be Ad-hoc grouping - I’m just controlling a group’s input. But I can kind of see the logic, just disappointing.

(I’m trying to do some automation that supports Alexa triggers, but not Sonos’s newer and seldom integrated Voice Assistant, so if Sonos only allows that via their app and Voice Assistant then I’m SOL I guess. Any other workarounds?)

… one of whose key features is that it does not have a cloud back-end or local side-doors. 

Any other workarounds?

The UPnP API possibly. 

It’s not Sonos’ limitation, it’s Alexa’s.  You can’t group on the fly with Echo devices either.  Grouping anything via Alexa is “Alexa, play <song/artist/album/station/playlist> in <group name>”.  There’s no command to say “Alexa, group kitchen with living room” or “Group TV with kitchen”.  

Right. AGAIN I’m not asking about ad hoc grouping. I’m asking for ways to get Alexa to play from a source to a group - instead of that source being Spotify or Tidal, it’s a line in.

Within the Sonos app, it is absolutely treated as simply changing the source (eg TV is listed right there with Spotify, Tidal, etc in the list of Music Sources), and they certainly could have allowed it to work that way through Alexa. If they didn’t treat it that way for Alexa inputs, that’s frustrating but good to know - the documentation here is often lacking.

(Incidentally, if I say “Alexa group kitchen with living room” it says “Ok, what would you like to call it?”)

Regardless of what you want to define it as, there is no way to play the TV or Line-In source to a group, or group to the TV or Line-in source, using Alexa.  Period.