Google Play will take up to 4 hours to finalise joining the Open Beta in the background. You can confirm this has completed by going to the Store listing for Whole Home Wi-Fi and you will see (Beta) added.
Update the app to the latest version and use it as normal
NOTE: If you have any problems seeing the new app, make sure you are only signed in with a single Google Account to ensure you have opted-in with the correct Google account. You can check in the Google Play app.