It would be cheaper to rent but they have no more cabanas to book on June 6th unless someone cancels but that's unlikely to happen.
We've had 15 people use one cabana and it worked out well. The cabana was just a holding place for most of the time as everyone stayed pretty busy with activities. They stayed in or near the cabana just long enough to eat or reapply sun screen.
Your in laws have paid for the first 8 guests ages 2 or older of the 15 total allowed. Starting with the ninth person, each person will pay $20 each upon arrival. As mentioned below, the additional $140 would be well spent in renting a second cabana. Unfortunately, Wednesday, June 6th has no day cabanas available to book.
If it's an additional party every person over 8 up to 15 is $20 it would be cheaper to rent another site
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