changing volume on grouped speakers

A named group is still just a group of devices. Not treated as a separate entity. Really just a convenience to select all the same devices easily. I had that same thought.  So, no that won’t work.

Yeah.  Would be a nice feature for them to add.  To allow grouping of speakers, and those groups to have their own volume slider.   Similar to a zone.    

Sonos does have zones, which sounds a lot like what you’re looking for, but it does not support Move or any portable speakers.  Likely because it the portable nature makes things more complicated 

https://support.sonos.com/en-us/article/set-up-a-zone-for-multiple-sonos-products

But I can see a need for a separate functionality just for this flavor of volume control.  Would it suffce to not even define subgroups, but just have a volume control for speakers that are not the source?  For example your Arc would be the source of TV or music (parent), and the 4 Moves would be the children.  It seems like it would meet most needs, but the parent/child relationship is not always easily apparent.

Yes that is correct.   The zone functionality does not include any of their portable speaker options.   Which seems silly, as they are all on the same wifi network as the non-portable ones.   🤷🏻‍♂️

Zones is an escaped corporate feature, not really designed for ‘home’ use. Most corporations don’t use battery powered/portable devices.