Is in the house rules that no guests are allowed? Did you ask permission to the host to bring the extra guests? AirBnB is based on trust and communication. You are staying at someone's place and the hosts has the right to know who is staying. Even if they didn't sleep there, they are using the bathroom, maybe they took a shower, etc... There are extra expenses in addition to safety concerns when people who is not part of the booking.
$500 seems abusive but maybe your host had problems with extra guest before. If you don't agree with the fee, decline it. Airbnb will get involved. You can offer to pay a bit extra (but not $500) for the costs of extra cleaning. It's important you keep the communication via Airbnb messages.