July/August is definitely going to run you a pretty penny. By the count of guests i'm guessing you'll need at least 4 or 5 rooms most likely. You can always get an OBC from a
travel agent, or you can book the whole thing yourself through Disney. If the TA will give an OBC for each room, i'd recommend booking through them as that will definitely add up. (We never use a TA, and for-go any OBC for the convenience of doing it ourselves and being in control of the reservation incase we need to make any sudden changes and god-forbid it is a weekend or holiday or our TA wants to ignore us. The maximum 200$ OBC they might give us isn't worth the headache of dealing with them for us, but with maybe 5 rooms that money would definitely be worth it-if they give OBC for each room booked.)
Before booking- definitely make sure of the budget- whether one person is paying for it, or if you are all splitting the fare- make sure everyone is comfortable with such a large, expensive, trip. As you know, its never just the base cruise fare, their are so many extras that can really run up the costs, and multiply quickly with 15 people.
When it comes to booking, make sure you have a list of each Guest's name who is sailing, their date of birth, and know exactly how you want the cabin arrangement to work. They price the children based on age, so not knowing at the time of booking can lead to extra unexpected charges later on, and know exactly how you want the staterooms arranged, as some cabins sleep different amounts of people.
Once you book, linking your reservations is no problem to ensure that you all dine together/near each other. Same for booking other excursions, activities, etc. as a group. Very easy with the Disney System.