Photo: Minimal test setup
Photo: Once a group created, you can still cast to a single Chromecast or to a group
Photo: The visualization in Chromecast app when casting to a group
Photo: Conveniently, you can adjust the volume of each Chromecast audio in a group, which is scaled when adjusting it.
In my experience however, the volume wasn't matched anymore after adjusting the general volume.
François Simond
Minimal test setup