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As @nik9669a suggests, usually it is just a matter of someone downloading the app, and connecting to your system, which can only be done while on your WiFi.
Yes. You can both log in with the same credentials. You shouldn't need to log in to use the Sonos system though if you are on the same wifi network as the Sonos system, but it will ensure you have access to all the features if you do log in.