I've played Modern Warfare 3 before with a guest and I redeemed my code just an hour before. It just keeps on saying "Can't sign in" and it's really annoying - can anyone help?
You can't sign in while a guest is also logged in. To get this to work you need to
Xbox Live won't let you sign in to Call of Duty Multiplayer with any profiles that aren't gold logged in. The "Guest" account technically isn't gold, so you're getting rejected for that. Adding a guest player is done on the Call of Duty server, not your Xbox.