@Brian68
Happens frequently Brian. You have two courses of action here...
1/..... Contact the guest and say that Airbnb's protection scheme is only available to declared guests, please amend the guest numbers to all guests who will be staying or you will reluctantly have to cancel their stay. sorry but too much of a risk!
2/......Get onto Airbnb and state, the guest has booked for, and you have approved 2 guest to stay. The guest now wants you to provide facilities for 4 guests, 2 of them undeclared. You do not feel comfortable continuing with this reservation as the guest wants conditions on the stay that are outside the scope of the booking and ask Airbnb to get the guest to cancel or perform a cancellation for and on behalf of the guest.
You will find either of those will get a quick response Brian.
Cheers.......Rob