After having planned 3, 9+ person vacations my best advise is:
Don't over plan - we would come to agreement in advance which park to visit on which day and we planned 1 meal that we would all meet up for ( typically in the park we were at that day for ease )
Before planning ask the others to do their homework and make a list of their Must Do's and If We Have Time Tos. You can probably find a lot of common ground.
Unless you have a 2 week vacation, it will be very hard to see and do it all. Pick your Must Do's and plan those first.