Single stock quantity but with option pricing differences

New to opencart and trying to work out how to configure options for the following scenario. I want to use the cart to enable people to book spaces to sail on a yacht owned by a sailing club. So a product is a particular day out on the water. The quantity is fixed - 8 people - but depending on whether it is a member booking, a young member, or a non-member as a guest, the pricing is different. As spaces fill up I need to subtract from stock quantity - but there is only one quantity I am tracking. So it is different from the more usual scenario where you track quantity separately by size, colour etc. Any steer on how to achieve this would be much appreciated.

Check here for more info on such:
https://www.opencart.com/index.php?rout ... ch=Booking
https://www.opencart.com/index.php?rout ... eservation
https://www.opencart.com/index.php?rout ... rch=Ticket