If at least one passenger pays to be aboard, then your boat is considered a “passenger vessel”. Period. If it is a typical recreational type of boat that is used for charter (and must also be endorsed for Coastwise Trade…), then it is considered an Uninspected Passenger Vessel. There is a long list of required items including country of manufacture, required gear, operational processes (such as drug testing), and so on. Maximum passenger count is six for vessels under 100 gross tons, and twelve for UPVs of 100-300 gross tons. Licensing is required for the captain. See more info here.
If the vessel is an Inspected Passenger Vessel, then the maximum passenger count is determined by the Coast Guard-issued Certificate of Inspection (COI).
I provide services to inspect your boat and your processes to make sure you are completely legal – these federal laws are enforced by the U.S. Coast Guard. Many of the items can be bought (like proper fire extinguishers) but many others (like a MARAD waiver, proper COD endorsement, proper FCC radio licensing or drug testing program) take some effort to make sure you comply.
Make sure you are legal – you do NOT want to get in trouble with the law out on the water.