You have to be on their WiFi. The same subnet. Otherwise, it isn’t possible. 

Thanks…I am on their wifi. But it keeps prompting me to login under their email. 

You shouldn’t need to log in, unless you’re adding a service connection, which will be your personal account data, and stay on their system when you leave. 

But you can play all the music using their Sonos all you want. 

Thanks Bruce. I’ve tried, but it can’t find any of my music. I see their system when I get on the app, but nothing of mine turns up on the search. 

It shouldn’t. You’re connecting to their speakers, which have their streaming services associated with them, not yours. Your account data is stored on your speakers, not in the controller or in a cloud system. You get access to the data stored on the speakers you are connected to. 

When I’m at my friend’s house, I use AirPlay 2 to send my stored music to their Sonos speakers, or just use their Spotify playlists. 

Thank you for the advice! Much appreciated.