They will need a mobile device with access to the local WiFi network subnet that Sonos is using and the correct/corresponding S2 (or S1) Sonos App (whichever App you use) connected to the ‘existing’ Sonos system, it will work without them requiring access to your Sonos account (obviously), or if they already have their own Sonos account/installed App then they can use that (whist connected to the ‘existing’ Sonos system).