I managed to play Grounded a couple of times, but then went to play yesterday and got the error message: "You must log in to an Xbox Live Account to play Grounded". I'm on PC, and hitting ok just sees the 'Logging in..' message, before it simply returns to the original error message. I tried everything, logging in and out of msoft account in the app, in my browser, uninstalling, reinstalling etc... but was confounded, as my partner can play just fine.
Then I realised, I'd updated to Bitdefender 2020 yesterday - so had my partner update hers, and now she can't play either. So - conclusion