Along with macOS High Sierra 10.13.6, Apple has released iTunes 12.8 which now supports AirPlay 2 multi-room audio and HomePod stereo pairs.

Use the AirPlay menu to easily group HomePod, Apple TV, and other AirPlay 2-enabled speakers and control what plays next.

More info and download link for iTunes 12.8 here.

MacDailyNews Take: AirPlay 2 goodness is now everywhere!