airplay is available to anyone on the network. doesn't matter if they're part of your home or not.
for third party speakers, there is no special apple sharing magic, they're just available. For apple speakers you have the option of limiting to people in your home, but it's easily changeable.
Third party speakers are also fairly limited in homekit, they just allow you to assign a room to it. It's used for siri commands to send audio to certain rooms.
The Sonos app can be used by anyone on the network. Things like adding services requires you to log-in to your Sonos account first, but selecting and playing music can be done by anyone on the network, using the Sonos app. I'm able to control my friends system when I visit them with no configuration changes, my phone is on their network, it shows their system, on my network, it shows mine. (I am unable to add or remove services at their house)
I highly recommend using the app, airplay can be a bit finicky.
with the app, you can add your music services, and then anyone with the app can control it. So you could start music from your iPhone, your wife can skip a song from her android, and either of you could change volume from your Mac or Windows PC.
you can also add multiple versions of a service, for example.. both yours and her Spotify, so you don't mess up the others algorithms.
Airplay from Mac also has the disadvantage of sending all system audio, so if you get new mail... DING ... right in the middle of your music.
