Can a person open a room and bring someone in?

You can't. According to Article 6 of the Measures for the Administration of Public Security in Hotel Industry, hotels must register when they receive passengers. At the time of registration, the passenger's identity documents shall be checked and truthfully registered according to the prescribed items. So once you check in, you can't sneak people into the room. All check-in guests must meet the conditions of "one person, one card", so you can't bring anyone in.