Step 1: Log out of the forums. Click your icon in the top right, click "Logout in the top right".
Step 2: Clear your browser cache, or open an incognito/private browser.
Step 3: Log in with the email address and password of your Gamertag that was Invited.
If you do not know the Gamertag associated with your email invite, please contact Microsoft Support at support.microsoft.com
If you need further support please create a ticket with the Sea of Thieves customer support team.