I had a guy show up with his girlfriend after making a reservation for only 1 person.. However, AirBNB said I could not change his reservation as it was already in progress.. Normally I would have just let this slide but water, electricity, etc., costs more for two people than for one.
I added an additional line in my house rules to cover this in the future. I suggest you do the same! Here is my verbage:
Additional guests not listed on your itinerary booking are strictly not allowed. If you need to add a guest please include them on the itinerary for initial booking or update your reservation before arriving. Guests not listed on your itinerary will, at owner's discretion, be charged an extra fee or not be granted access to the property.
I feel this is necessary for both financial and for safety reasons as, at least in my situation, I am still present on the property and anyone I am unaware of may pose a problem to me or my safety.