@Allison2
I just had my first attempted sneak-in.
Fortunately and largely due to your advice I think I might have sorted it out.
So long as you have an extra person charge and photo evidence they ought to rule on your behalf.
Extra Person Charge: I don't want extra persons, so I set a detterent extra person charge of £100 per extra person per night over 8 people.
House Rules: I stated No Unauthorised guests.
The group arrived in 3 smaller groups, the last group was almost unseen but we saw that they made a group of 9. We allowed them to stay as we were uncertain whether one or more guest might leave to go elsewhere, but in the morning there were still 9. They walked off from our place so I shot a short video showing the 9 people.
The Airbnb advice in this situation going via 'Help' in the guest booking was to make a booking change instead of a resolution request, so rather than just send a booking request I contacted the guest via AirBnb message system, appologised that I was the bearer of bad news and that because they were a group of 9 they had an unauthorised guest and we did not allow unauthorised guests. I advised that AirBnb's advice in this situation was to send a booking change and "did they want me to book one unauthorised guest for one night or for two to include the forthcoming night? If two, there was additional bedding in the wardrobe."
We have no extra bed but guessed the guest slept on a sofa.
The guest replied to say they knew about the extra guest and possible charges, the guest surprised them with his arrival, but they didn't know what to do so tried to hide him. They agreed to the extra charges.
It's just a pity that they tried to hide the additional guest.
Edit: First hurdle... I can't make a 'booking change' as I am at maximum occupancy and cannot +Add extra guests dispite the listed charges. @Allison2 You were right!! Payment request sent !