Private Group Bookings

I would like to be able to be able to charge for a Nordic Spa service, that has a maximum capacity of 6 people and a minimum of 2 people. I would like to be able to charge per person, but once the minimum number of people has been met, the remaining slots then become unavailable so that it is a private booking for that party.

What I want to avoid is that for eample a group of 4 book a Hot Tub etc. and then 2 strangers can book into and join that timeslot.