Teresa, we did the same thing recently and here is what we learned
1) Be carefull about the wording in a group contract. If the price on the cruise goes down you want them to lower your rates as well. You do not want to hear them say 'well, you have a group contract and we can't'. Rates fly all over the place, leave your options open. As for the 'free' cruise per 8 cabins, that is usually given at the worst room category and you still have to pay port fees so it really isn't that much.
2) Look at getting off the boat for the wedding, the cruise lines are just there to make money and the rates are expensive. You will be on the boat for 7 days, goo someplace fresh and st. Johns/Thomas has great locations and good planners.
good luck to you and enjoy