[Update: iTunes 12.8 was originally released in July, even though the release document linked above was dated September 12th. – Bryan]
Apple’s sparse patch notes:
iTunes 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.
You can down the update through Updates in the Mac App Store for macOS High Sierra. If you’re running macOS Mojave betas, you can check Software Updates in System Preferences.
The standalone update on Apple’s site is a 275.9MB download.