Just to be clear, it's not 8 passengers, it's 8 cabins.
As for getting together the group, I agree that it's always best to contact a cruise specialist as they have all the information and resources you need.
First rule of thumb that I always tell my group leaders; do not present a bunch of options to your group - you will get as many difference answers as there are options and in trying to satisfy everyone, you usually end up causing more problems. It's always best to make the decisions and then let the group know what is being offered - that way they have one choice to make - whether to go or not. Makes it much easier on you, as the group leader.
Second rule of thumb, once you've made the decision, then turn everything over to the cruise specialist and let them do their job. Notify everyone that they are to contact the agent and not you. This will save you a huge amount of hassle. They should also have the ability to send out electronic 'cruise sheets' so everyone will be able to see everything about the cruise, the ship, the itinerary, and the price. For example, we send one out where you can click on the port and it'll also provide information about what there is to see and do there.
Bottom line is, when it comes to working with groups, the more information they get from the agent, the less they will want to contact the group leader. And this is a good thing if you want to save your sanity.
I've been doing groups for almost 12 years of all sizes from 6 to 8 cabins up to 365 people. So trust me when I say that should you book this yourself through the cruise line and not use an agent, you will thoroughly regret it. Everyone will be contacting you with all sorts of questions and problems, and you'll end up with so much hassle that you'll end up not enjoying your cruise at all. Plus, you may not get all the amenities you're entitled to as often the cruise lines will benefit due to your lack of experience.
Disney is a great cruise lines for families and for groups. We've always enjoyed working with them because they help us provide a wealth of information and services to our groups.