Along with macOS High Sierra 10.13.6, Apple released iTunes 12.8 with AirPlay 2 support. The update gives iTunes the ability to see and play to paired HomePod speaker sets, as well as play to multiple AirPlay 2 speaker groups.
AirPlay 2 lets you stream music to multiple speakers throughout your house. Now that macOS and iTunes have AirPlay 2 support all of Apple’s product lines support the feature.
iTunes 12.8 is a free update available through the App Store app on your Mac. Just go to Apple menu > App Store, then click the Updates tab.