I did a group of 16 once. Stayed at OKW, did not get a Grand Villa. We had 2- 2 bedroom lockoff units and a one bedroom unit.
Breakdown was:
Two Bedroom #1: Grandma, Grandpa and 10yo Grandson were in the side with King bed, grandson slept on fold out. His parents slept in the studio part.
Two bedroom #2: Mom, Dad and 4 kids (Incl 1 infant) on King side. Another Mom and Dad and their kid in that studio part.
One bedroom unit: Myself and a brother to one of the parents. We could have made do in a studio, but I wanted the jacuzzi and washer/dryer. My points, my call.
For many, it was their first WDW trip. It was nice, we did break into groups some of the time, but usually went to the same park that day. Trying to do stuff all together in one big group is like herding cats. Give everyone their space, give them all the info, plan a couple nice meals together, pick a park of the day, then let each sub group do whatever they want for fastpasses, meet & greets and so forth, or you will drive each other crazy. Oh, and make sure each family group has an EASY to understand guidebook with basic info, don;t throw too bog them down with TOO MUCH info, the condensed version is good. If you get too detailed a guide book for first time visitors, they are like deer in headlights. Most planning, like picking a restaurant and so forth, will likely be left to you, unless they are also experienced Mouseketeers. Just get an idea of their budget needs and restraints, and ask the kids their favorite characters, when planning character meals.
We grilled at the pool one night (we had a car for Publix runs). We did 8 people at Cindy's breakfast, it was nice, a big group at Prime Time, (they fit us all in one room) and didn't plan a meal at AK, everyone did their own thing. At EPCOT, part of the group did Chefs de France, the other part wanted pasta and did Tutto Italia. As always, book as much as far out as possible. Mosts breakfast were in the rooms, or coffee and danish in the parks.
Good luck!