1st of all, aside from a few meals, and maybe picking a park you will all go to on the same day each day, don't even bother trying to make plans for everybody. Everyone's eyes will gravitate to something different the moment they step through the gate of any park, and some people will tucker out sooner than others and might need to take 5 on a bench or even go back to the resort for a nap or swim. Try to get maps of each park, and pick places and times where you expect to meet for a meal or to leave or whatever. Then, buy a couple of planning books and encourage everyone to read them or get their own, so THEY can decide what THEY want to do.
Occasionally, the wants or needs of most of you, if not all of you, will coincide, and that will make that time spent together all the more valuable. Besides, one of the neat things about large groups is when you stumble upon each other after splitting up into smaller groups. It's also gives you fun mealtime conversation when you've each had different little stories to tell.
As far as rental cars go, there is a National and/or Alamo rental facility on-site near the entrance to the Magic Kingdom. You can rent a car whenever you're ready for one. You can probably opt to drop it off at the airport if that winds up being more convenient. And, from what I understand, they'll even pick you up at your hotel and bring you to the facility to get your rental car. I think Shades of Green would be less than a 10 minute drive to their facility. If my memory serves, Shades of Green guests all have to go to the TTE in order to go anywhere else they want to go (no buses go from SoG directly to a theme park or Downtown Disney). If I'm wrong, my apologies, but if I'm right, it might be more convenient to have the rental car the entire vacation, if your budget permits it.
***EDIT***
I found out some info regarding the bus schedules, from allearsnet.com
What transportation is available at SoG?
SoG has two bus schedules. Bus loading and unloading is located on the lower level below the lobby. There is an elevator to the Bus Stop located outside the lobby in the Port Cochere.
Shades of Green room key is required to ride these buses.
Check upon arrival for the Extra Magic Hour bus scheule.
Bus A
* Departs Shades of Green every 15 minutes starting at 0800.
* Departs the Ticket Transportation Center every 15 minutes
* On this route you will get to the Magic Kingdom or Epcot (Via monorail), and to the Downtown Disney Resort area via bus from space 26 which runs every half hour until 2300.
* Drop off locations for Downtown Disney are either at the Wyndham Palace Resort or the Hilton Resort.
* The last bus will depart the Ticket Transportation Center 2 hours and 15 minutes after the Magic Kingdom closes.
Bus B
* Departs Shades of Green every half hour starting at 0800 to Disney's Animal Kingdom, Blizzard Beach and Disney's-MGM Studios.
* Departs Animal Kingdom 15 minutes and 45 minutes after each hour (Space #7)
* Departs Blizzard Beach 30 minutes after the hour and at the top of the hour (Space #1)
* Departs Disney MGM Studios 45 minutes and 15 minutes after the hour (Space #6).
* The last bus for Animal Kingdom and the Studios will be 2 hours and 15 minutes after each park closes. The last bus for Blizzard Beach will be 2 hours after the park closes.
Also, it is apparently about a 10-minute walk between the SoG resort and monorail resorts The Grand Floridian and the Polynesian. If you just missed a bus to your destination, it might save time to walk to one of these resorts and jump on a bus from there.
