Just to uphold the friendly nature of our board here, I want to point out that OP did not say he wanted to sneak an unregistered guest in, but that he would want to check in an additional guest. So, I guess it would depend on the hotel and the room. If the room is rated for the number of people that show up, I think they'd let you check them in, but would definitely charge you the rate for the number of people there.