If you're doing 2 player and it needs the second controller signed into an Xbox account, you would press the center button on the controller to open up the guide for that controller and log in an account. If you've never synced the controller to your system you would do what Cat says above, press the center button to turn on the controller, then the button up by the port on the top of the controller, then press the button on the xbox next to the second memory card slot, and it should sync it, then you can try to sign on the second players account using that controller.
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