HereWeGo ........ After thinking about it, seeing as there are people out there playing multiplayer on Xbox...I think it's definitely an account problem.
So setup a new account on your Xbox -
Press the Xbox button to open the guide.
Navigate to your profile and system settings.
Select "Add or switch" and then "Add new".
Enter the email address associated with your Microsoft account.
Follow the on-screen instructions to sign in and configure your account settings.
Make sure Ground 2 is closed down fully (not quick resumable) and then relaunch the game ... once there switch the profile to this new account you setup and bingo it should work!
I just tried it now and can access Multiplayer under this new account.........