What problem? SONOS players, BOOST, and controllers must be on the same network segment. “Guest” is. a separate network segment that is normally used to allow house guests access to the external Internet, while blocking access to local devices, such as SONOS, computers, thermostats, cameras, etc.

Boost won’t separate anyone from anything. It’s a network bridge device, not a router. 

I would have understood the question better if it was worded like this: “Would a guest network work prevent guests accessing Sonos (on Sonosnet or regular wifi)”? The answer would have been a straight “yes”.