You can go to the trip and request money, stating that it is for extra guests.
You can ask Airbnb also to mediate, saying that she needs to amend / accept the extra money or they (airbnb) should cancel the reservation without penalty for you, as the guest will be breaking your house rules.
In your reservations section there is a blue link which says "change or cancel reservation" in there is the option to change guest numbers, dates etc... Also, is cost.
You can amend the guest quantity, but also check the cost is updated to your listing requirement.