I have gone with a group of 9 before. I was involved in some planning of my brother and sister-in-laws wedding planning. I was not with as large of a group as others who have posted, but they have already given you some sage advice. Do not get upset if some people want to do one thing and the rest want to do another.
I would suggest planning at least the theme parks that you want to go to each day and just suggest meeting times and places at each park and let people wander at will. We tried to do the whole group together and it worked out well for the most part. If there was ever a ride that someone didn't want to go on, they just sat outside and waited for us or browsed through a nearby gift shop.
As far as dining reservations, I would make sure you have those ASAP! You can book so far in advance right now, popular restaurants are getting filled up with the dining packages becoming more popular, affordable, and sometimes free depending on your package. If there are picky eaters, try to stay to basic restaurants:
-Chef Mickey's
-any of the Italian restaurants (Mama Melrose, Tony's or Alfredo's)
-Biergarten (Germany)
-Rose and Crown (United Kingdom)
-50's Prime Time Cafe
There were some different dietary needs in my group, so we were able to work everything out. For the most part, it worked out well. Just remember you are in the Happiest Place On Earth and to have a good time!