On our one (1) BVI charter
2 years ago - 3 couples - we split the charter
costs equally, and those of us who drank worked out those purchases amongst ourselves. Mooring
and other common expenses - we kept notes and receipts and split later, and each couple paid individually for dining out, entertainment, etc. It all worked out... but it helped that we're all good friends. Still, KEEP RECORDS. People forget, especially when there's too much fun happening.
I've also helped manage an annual sailing get-together which draws in 7 to 11 boats from Canada
and the US. We often arrange marina deals, prepaid dinners, and have some small awards. The financials are a bit messier, and it's even more important to set expectations, explain in advance how costs are going to be shared, and KEEP RECORDS. I had an online registration
system for signing up, printed out spreadsheets for tracking who's paid, and kept a steno pad aboard listing all transactions.