Revisiting this as I received my Sonos speakers today. Is there a slider for the group or another solution? Adjusting volume in macOS now requires me to click + and - buttons while there are sliders for the individual Sonos speakers.
Also, when I use the iPhone volume buttons in the Roon app it does not adjust the group but only one of the Sonos stereo pairs of the Roon group.
-edit-
I see I am not the only one requesting this feature.