You have a bunch of people going. Using a travel agent who specializes in cruises is a good idea. Many of our Carnival fans have developed good relationships with their PVP at Carnival. However it is not cheaper to use Carnival instead of an agent. Actually it is supposed to be a level playing field, with the prices being the same. Sometimes one can find a special deal at Carnival, but it is supposed to be available to the agents too. Actually, if you check around you might find an agent who has blocked out group space on the cruise you want. In that case it is usually going to be less expensive than the going rate for individual bookings.
Bear in mind that Carnival recently changed their cruise fare framework. They now have an "Early Saver" rate which can offer savings but also has restrictions such as a non-refundable deposit. Be careful of which rate plan you book under. Read about Early Saver here