I would agree on suggestion of using a travel agent...an actual person you can talk with.
You can set them up as point of contact for everyone in group to provide their information and payments, travel documents, everything. I've read some painful experiences here of people trying to coordinate a big group. That takes the awkwardness and responsibility for that off of your shoulders.
Also, I believe the discount, like one person free or something, starts at 8 cabins through Carnival Groups dept. You can deal with the groups dept yourself, but i would assume a TA would best help herd the cats and remind everyone of deadlines, etc.