1st off, $800 or so gone on day 1. (Provisions beer/ rum).
The purser can divide and leave some in a container(small bag, coffee can) in salon which is accessible by all. There will be lots of things in salon.... or down below somewhere. Often times when all going ashore, purser takes kitty.
If someone buys supplies, when back at boat, can reimburse self... Honor system... If can't trust them, should not have invited them.