Hi Mel,
Interesting situation .
Last week we had couple stay from Europe .
As I was not going to be at home when guests arrived we had organised for them to use key we hid. Booking was for a couple only .
On my arrival home the usual meet & greet, I found their son lying on a bed to be staying one night.
I was taken back by comment and thought quickly to say - ' that is great your son could join you for a night - are you going to contact Airbnb to advise change of booking to 3 guests ? '
That worked as they advised as new to Airbnb & do not know how to alter booking they would pay the 'extra guest fee' for his night stay in cash.
They were here for 9 nights and son here 3 diffetent nights and they paid cash for those nights he was here .
Felt ok as was another towel, sheets, water , soap etc..
Wording like I did, they had to take ownership of the change of their booking.
Hope this assists u in your situation.
Annie