proportional/split billing for group play

If 20 people show up for a doubles group play where they are using 4 courts, I don't want them to pay full price, I want them to pay proportional to the number of spots (in this case, 16). But there is no way (other than manual) to give the discount for group play. I believe it's possible for private bookings (e.g. 5 people can book one court and pay "proportionally" so they are not all paying full price because they are sharing the court five-ways), but I don't believe this is possible for group play.