We have had it happen. It depends on how many people. If the room can accommodate them, we tell them the extra charge $20 per night per person. If the room can't handle it, we ask them to come up with a solution. No one is legally allowed to stay on the floor and I have a legal maximum occupancy.
We have had one where they tried to hide 2 people from me. We didn't charge, but we also didn't provide services. In fact, we sort of delighted in making it extremely difficult for them. For example, we remade the room and put all the furniture back, so they would have to move everything. They didn't get breakfast, sheets or even towels. The stupidest part, had they told me, I could have made a bed up for them, they could have had breakfast, clean sheets, towels. The biggest irony? It appears they bought the air mattress and sheets.... my extra charge, would have been cheaper!