If this is a concern, don’t share that wifi network password with your guests. Sonos doesn’t provide any security over whatever your network is configured for.
They are accessing sonos via the guest network. Sonos is not on the guest network.
@Valvequeen,
Most ‘guest’ networks have an ‘isolation’ option in the router settings - in the case of my own router/mesh setup, I can give guests just internet access only and no access to any other device (wired or wireless) on the network, or I can give them access to individually chosen devices by MAC address - so the fact your ‘guest’ WiFi provides access to all Sonos devices is very much down to your router, rather than a Sonos issue.