How do I "Use the same Apple ID for every computer or device on your Home Sharing network"

I'm trying to share two iTunes libraries from two different computers with my Apple TV. All steps in the instructions are straightforward except one. I'm told to "Use the same Apple ID for every computer or device on your Home Sharing network." But I don't know how to do that. There are no instructions on how that is done. I'm sharing one of the libraries from one of the computers successfully. So apparently that computer has the correct Apple ID but the other computer does not. The other computer belongs to my wife and she has her own Apple ID. If I change the Apple ID on her computer to my Apple ID am I going to screw up everything on her computer?