Sonos Group Master - is it broken? How to adjust group volume?

I don’t see the master attribute anymore for my Sonos and whilst I can successfully join a group of speakers I can find no way to adjust the volume of the group as a whole.

Can this still be done?

This will join them…

sonos_group_some_speakers:
  alias: Sonos Some Speakers
  sequence:
    - service: sonos.join
      data:
        master: media_player.speaker1
        entity_id:
          - media_player.speaker1
          - media_player.speaker2

…but this only changes the volume of speaker1 (which is set as the master)

      - service: media_player.volume_set
        data_template:
          entity_id: media_player.speaker1
          volume_level: 0.5