I think only one instance of the Google assistant (Google Home) can be used with a Sonos household - The answer would be to use separate Sonos households - so you need to setup 4 separate Sonos systems with their own controllers - best to use different Sonos accounts and each person links their own Google Home and their own Spotify accounts to both their Sonos system and their own Google Home.

Wow - thanks for your fast reply!

Just to make sure I understand:

1. Everybody downloads Google Home and create their own homes containing the four speakers.
2. Everybody sets up their own google assistant connected to their Google Home. 
3. Everybody sets up their own sonos account and connect it to their won spotify and Google Assistant accounts.

Did I miss a step and did I understand properly?

Not quite. Each user gets their own Sonos speaker(s) - and each one is setup using their own mobile and Sonos controller App - so they each only see their own Sonos system with their own speaker. A bit like having their own stereo in their own room. Each person controls their own speaker(s) with their own phone, have their own Google Home and their own Spotify account. All systems can run on the same WiFi network, but each are entirely separate entities.

Hope that assists. 

You need to add your family members to your home in the Google Home app. No need for separate Sonos systems and/or Google homes.

First, make sure all your family members have a Google Account in order to use Google services like Home and Assistant.

Then, follow this link: https://support.google.com/googlenest/answer/9155535?hl=en#zippy=%2Cinvite-members-to-a-home Scroll down to the section titled “Invite members to a home” and follow the steps. Your family members will have to install the Google Home app on their phones to accept the invite they will receive per email.

After all this, every single family member will have to set up Voice Match by following the instructions on this website: https://support.google.com/googlenest/answer/7342711?hl=en&co=GENIE.Platform%3DAndroid

This is so the Google Assistant can tell you apart by recognising your voice and play music from each person’s Spotify account separately. Each one of your family members will have to do this using their phone with their Google Account logged in to the Google Home app.

Each family member will also have to link their Spotify account to their Google Assistant. You’ll find instructions on how to do this here: https://support.google.com/googlenest/answer/7551876?hl=en&co=GENIE.Platform%3DAndroid

After you’ve done all that, everything should work.

Note: When setting up Google Assistant on Sonos speakers (not Voice Match, but adding Google Assistant to the speaker itself for the first time) that don’t have voice control enabled yet, always make sure you do this using your own account. Otherwise speakers might show up in the Google Home app as duplicates. 

Hi again

Thank you for both of your suggestions!

I tried your method @JamesDan but got stuck in the process. I made sure my daughter had a Google account. Then I followed your first link to invite her to my Home. That worked fine.

Then I followed your second link to have her set up Google Voice Assistant. When she followed the instructions she encountered a problem. The app told her that she had to add a devise in the Google Home app to be able to configure Voice Match. And all she could do was to exit the setup without setting up Voice Match.

I then fooled around with the sonos app. I used it to search for sonos devices. I found our four speakers and added them one at a time to our share home (I already had them in the home). And then we tried to set up Voice Match again. However we got stuck the exact same spot with the app telling us, that we have to add devices to the Home app in order to set up Voice Match.

Any idea what I’m doing wrong and how to proceed?

Cheers!