HI @Alan Mead, thanks for reaching out to the Sonos community and letting us know about your concern. You need to download the S1 app to be able to connect to the S1 system. If you have any questions or concerns, please do let us know. We'll be glad to assist you.

I’m visiting a friend with an S1 system. No big deal to download and run the S1 app without needing to touch or change your S2 app. 

Make very sure you don’t accidentally update their system to S2, else you won’t be invited back again!