Cruise lines will pay a commission. If I am booking a cruise for the client, the client pays me $100 initially, and nothing further, to include the others going on the cruise.
I normally go on group cruises to ensure the group receives the amenities I have set up for them. Group cruises are, on most ships, 5 cabins or more. I am able to provide amenities for the groups through the ships staff.
Sometimes, the ship personnel will make a mistake and not provide the amenities to every cabin within the group. That's why I go to ensure those amenities are delivered and provided.