I can't recall the number for Disney, but for other lines the magical number is 8 - to make up a group.
The key to Disney is booking early - as soon as the itinerary is out. This makes it hard when you are trying to manage a group. Are you able to pick a date and say "this is when we are going" are you dependant on agreement from all parties?
If the later, get group consensus on a date outside the available booking.
For example, I'm not sure yet if you can book for 2015. If not, figure out a sail date that works for your group. Then have an agent ready to get you your cabins on that sailing as soon as itineraries are announced. Especially if you are trying get adjoining/special needs or cabins within a similar proximity - this will give you the most availability.
You will also get the best price this way. If there are sales/specials/price drops - your agent can grab this for you before final payment.
I've been on two Disney cruises, and both times have booked early, and then watched the prices steadily go up until sail date. Yes, just before sail date - there is a 3 month window (I think) when people who have put down deposits don't follow through on final payment, and you may see price drops at this time. It may be easy to get a few cabins together, but not an entire group. After that, prices will go up again.