You need to log out of your current system in the Sonos App. Log in to Sonos under a different email not currently associated with Sonos. Select create a new Sonos using a speaker that has been factory reset or recently purchased.

Yes, as long as the other family members are not connected to your office WiFi, they definitely won’t have access.

At the Office (assuming no Sonos system is in place already) just reset the Sonos App on your mobile device to create your new office Sonos system on the local WiFi network.

On return Home for the first time after that office setup, connect your mobile to your Home WiFi and open the Sonos App and connect back to your Home Sonos System. After doing that one-time, as you then later switch between Home & Office WiFi networks, the Sonos App will automatically switch to the local Sonos system.

What @Ken_Griffiths says us possible. I answered under the assumption that your office was also in your home as many people work from home these days. Therefore you most likely only have one WiFi network. 

Thanks a lot for your kind reply. 

The office is in another building with a different WiFi network. However I wonder if wouldn’t be an alternative to create another Sonos account with another email in order to be sure that the 2 systems are surely separated. May I do that using the same APP? would it be more difficult to be managed ?

many thanks again!

If you want the Sonos App to automatically switch between the two systems (Home & Office), you are best to use the same Sonos account for both systems (often referred to as Sonos ‘Households’).

Each Household is held ‘separately’ in your online Sonos account and the App will auto-switch between those Households merely by connecting the mobile device to their corresponding WiFi network - there is no opportunity for either systems to be accessed from the ‘other’ network.

Setting it up in the way described, will make switching between systems much easier… but if the family members mentioned do access the office network, you would be best to use a separate Sonos account with completely different login credentials, but switching between Systems/Households then becomes much more difficult.