My friend moved across the country recently, and we thought this would be a fun way to stay in touch for now. He accidentally input the login to an older account of his, and due to his change in IP address, his Microsoft login failed, and asked for verification. He no longer has access to that email address, and as it's his older account, his phone is no longer associated either for verification. Even after reinstalling the game, it remembers his old login, and it returns to the verification screen immediately. We tried clearing the computer's temporary files too, but to no avail. 

Having a similar issue, can't clear it to log in a new microsoft account.

Hello! I'm sorry you both have had issues with your Microsoft accounts and accessing Grounded.  Players should be able to change who signs in at the login screen; after pressing Sign In under their name, there is a Sign In With A Different Account link below. Once you click that, anyone should be able to sign into the game. 

If this is an Xbox Live specific account issue, there isn't much we can do on our end, and we recommend you reach out to Xbox Support.