@Bryan237 Airbnb regards the individual who places a booking as the primary guest, but they don't have an official distinction about how secondary guests are counted and registered. The primary guest can't transfer responsibility for the booking (including damage claims and reviews) to another member of the group if they leave early, but they don't require the primary guests to submit any data on who the other members of the group are. So if one is swapped out for another person during the stay, I don't think it makes any difference to them.
However, it does make a difference in terms of how a host would deal with the situation. Let's say you have a 2 bedroom house and a booking for 4 delightfully amorous guests. If one of the bedrooms is vacated after the first night of passion and the primary host wants to invite a different couple to the love nest, they are probably going to need a fresh set of linens and towels to replace the wet and stinky ones. In this scenario, your costs would be about the same as if you'd had a 6-person booking, even though only 4 were on the premises at any given time. So if you have an Extra Person Fee, it would be appropriate for that booking to be charged as 6.
However, if you believe you can accommodate a rotating cast of guests without any extra cleaning or supplies, and your Extra Person Charge is based solely on occupancy, you might think of this less in terms of cost than in terms of risk. I wouldn't personally have much trust in a group that seems disorganized and unstable, as the passers-through have no personal stake in the way the home is left at checkout. So my personal approach would be to require a separate booking for each unique combination of guests, and let the group decide on the amount of intervention they would like in the changeover process between each booking.
