Deal with a travel agent. First, though see how many people are definitely going to go. The amount to book a group is 8 cabins. Deposit is usually with 30 days . and I say that because if people are really interested they will come with the initial deposit. As a
travel agent, and 15 years experience I inform my client , is to get $100 PP UP FRONT as the initial deposit to get the group to commit. Each cruise line has what they call points or amenities for each sailing date, and WITH 8 CABINS, NOT ONLY WILL ONE BERTH BE FREE (LESS PORT FEES AND GOVT. FEES) but you might get, ship board credit per cabin, 1 Cat. up grade, $$ off per cabin, bottle of wine or fruit basket per cabin. It depends on the cruise line, sailing date and amount of sailing days.
If you have atlest 5 to 6 couples then contact your travel agent and let her or him contact the cruise line, as many agencies belong to a corsortium that might provide A better price for your group, then dealing directly with Carnival or a cruise line. I know because we are a Virtuoso agency, we are considered a select agency with various cruise lines and we get some great rates and deals for our clients.
Deal with a travel agent, and not the internet, because you will be able to deal with them
Also find out how long of a time you will have to get all 8 cabins booked, Once the agent has booked group space for you, she or he will tell you when the first deposit is required and how much, then a 2nd deposit will be required to make the actual deposit of $250 PP that is normally required when a cabin , normally with Carnival and most cruise lines now ( a deposit of $250 PP is required with 24 hours to 3 days.)
Remember the balance is due usually 70 -75 days prior to sailing date. So I advise you to get the full deposit of $250 PP as soon as possible, then tell them that if they want they can pay on it weekly or monthly. This way there won't be a problem of not having all the money collected when the balance is due.
Hopefully this will help you .