You've gotten some great information here, especially about booking with an agent. Always the best thing to do. And yes, if you book it directly with Carnival, you can transfer the booking to an agent.
All the have to do is cross-reference all the reservation numbers and everyone will be assigned the same table for dinner. If they do not have a table big enough for your party, then they'll make sure they assign you two tables next to each other.
Best advice is that once everyone gets their sail-n-sign passes, check the table numbers to make sure you got the same table or two tables next to each other. To be on the safe side, once you get on the ship, go to the main dining room and verify where the table is or that the two tables are together. If there are any problems/questions, then that's the best time to talk to the Matre'd, who will be there just to handle any problems since people are always wanting to change seating times. But don't wait to do this because then it becomes more difficult for them to solve any problems that might arise.