The iOS 11.4 update brings AirPlay 2 to your iPhone and iPad and a collection of new Apple Music features.
AirPlay 2 is a new and improved multi-room speaker setup that allows you to use built-in controls and Siri to choose where to listen to music. There is support for existing AirPlay speakers, new AirPlay 2 speakers and even your Apple TV as a speaker. This also enables using two HomePods in Stereo mode.
Apple Music also supports sharing so that friends can see what you are listening to. You have control over making listening public or private since the introduction of iOS 11. A new shared playlist style feature lets a friend add to an existing playlist while you are on a shared set of AirPlay speakers.