I do use the default price per person, and have set the price for additional people but I also use the weekend price to increase the price at weekends. Airbnb isn't applying the weekend rate to weekend bookings. Its only using the weekend price for the first person in the group, for the rest its applying the mid week base price.
I can't use the option of one price for the base booking for the minimum group size and the charge extra for additional people because Airbnb's system sets a limit on group sizes of 15. While the base price for my property is the price for a group of 20 people with the option of booking groups up to a maximum of 32.
So I took advice from the Airbnb on the best way of setting up the price details. Which is why the property is priced per person, with it set out clearly in the description that guests should ask me for a special offer based on the size of the group. Unfortunately even when they do that if its for a weekend the price they are being shown is incorrect. So I spend a lot of time having to apologise and explain all the problems with Airbnb's systems.
It would be nice if they just increased the maximum group size from 15.