Adding Guest Xbox live user requires Both Controllers
This is ridiculous trying to get a guest user added to play with a group of friends online. It took almost 45 minutes until we figured out the correct sequence to add a guest to play horde on XBOX live.
Click Y to add Guest
Click A to accept
Click Y again to add Guest
Select Guest Xbox Live user.
FYI At no point should you select to sign in a secondary account local on the xbox. This kept throwing us off. What as the point of having to select twice.