When i try to add a guest it says it does not have a valid xbox live. Ive never had that problem or do i do something else i havent tryed to use a guest in a while.
You can take 2 types of guests online , i thought.
1- An account that you created on your xbox wich has to be gold for online play even as a guest.
2-or just the guest account that you should be able to select (the bottom one). should be called just guest.
with that acount you can always play as guest if the game provides guest play online.