I just did this for 10 of us this past August~
First off nail down the dates you are going, and then where to stay.
What I did first was go online and got differnt quotes for differnt hotels and dates then we all compared. So at least if they can give you a ballpark time frame you can do this, it was a big help~
Also you will need to figure in traveling, we flew so we did a package and it was all included.
We thought about all the different places, and settled on All Star Movies that way it fit everyones budget!
As far as planning out parks and eating, this board was a BIG help, I did go other disney sites & boards but this one helped me soooooo much!
I also told everyone to get some type of disney guide book so they could "help" in the planning, like anywhere they really wanted to eat or a show they wanted to see? This way nobody felt left out!
Once I had that from everyone I started planning, I made an agenda (anal I know) but it worked! Really well!!!!
I figured what day for what park, and what time and where we would eat that day, now dont get me wrong I was no drill sgt! BUT it really did help, that way every morning you didnt waste an hour doing the "where do you want go to go? I dont care where do you want to go?" that drives me crazy!
So this way if somebody wanted to go on there own they could and then we met up for dinner!
We did do all 4 parks, no water parks this time~
I guess this is out if the norm, but for the most part our group stayed togther, with the exception of my MIL, and that is another post all its own, but we had a blast!
And on our next to the last we planned a "Do what you want day" where everybody went on there own.

That way if there was something you wanted to do again you went back or something we missed you wanted to see you could go~ But that in mind we did decied on that day we would meet up later in DTD for dinner at RainForest Cafe~
We had a blast at all of our dinners, and had a great time!
I always tried to make dinner ressie about the same time everyday, it was easy to remember and kinda plan around. We had the dining plan, which was a big plus for us.
We only had one major "Melt Down" and you guessed it, my dear MIL
Other than that we all got along great and had a blast!
And they did laugh at me at first with my "Agenda" but as they will all tell you now, they loved it after we got there and could see how overwhelming it could be!
Sorry so long but hope I helped ya little!!!
Have a great time planning
Dana