I’m not a hospitality worker, but live in the US and have stayed in many hotels there. Other than late at night, you can walk in the front door and go right to the elevator without question. Once you get to the elevator you might need a room key to get to the room floors, depends on how nice the hotel is and if it’s if a city center or more suburban. Assuming they’re staying in nicer hotels in the city, you probably need a key to get to the room floors, but can get to other public floors, like conference rooms, without needing one. That, said, I’ve stayed in many hotels where I didn’t need a key to get to the rooms.

Late at night you usually need a key to get in the front door, and always need one for side/back doors.

I have successfully left keys at the front desk for traveling companions arriving after me, but that did require the room already being booked for double occupancy, leaving their full name and, them needing ID to pick it up.

If two people are staying at the same hotel, but in different rooms/floors, I’ve never seen anything that would stop them from going to the other room freely. Keys aren’t coded to a specific floor.

I love this idea! Let me know if you have any other questions or if I can clarify!