@Fiona470
The base price is the price for all guests (1 to maximum amount of guests).
If you charge a fee for additional guest, then for each additional guest (above the treshold set) that fee is charged above the base price. You easily check if works like expected by simulating a booking on your own listing.(or use the pricee calculator on the calender)
https://www.airbnb.com/users/97908753/listings
example
If baseprice is set to 30,- and fee above 2nd guest is set to 10,-, then the total for four guests is: 50,-
Please note discounts are only calculated regarding the baseprice.