We did this a few years ago with my husbands family. We had 11 people ranging in ages from 4-60+. My MIL put me in charge of planning out our trip and organizing things for us. Everyone in the family had been to WDW at one time or another over the years, so on weekend while we were all up at the cabin about 6 months before our trip, I made up a survey for everyone to fill out. I asked questions like what were 3 things you absolutely wanted to do and 3 things you absolutely didn't for each park. What restaurants do you absolutely want to go to etc. Then I pooled everyone's answers and was able to do things like planning ADR's and park days based on those. It worked great. It was just helpful to know in advance the kinds of things that we could just skip over and the things we just couldn't miss.
We also had our FP runners. My DH and BIL would run to stuff while we were in lines or eating to get those. Make sure to get those for the important harder to get into rides first and then move your way down the list. We mostly stuck together because the IL's paid for the trip and that is what they wanted, but we did split up some days. There was one day when they were going to Typhoon Lagoon and neither my DD or I wanted to go. We spent the day at the pool while the rest of them went to the water park. There was another day where my DH and I were able to go to DTD for a few hours by ourselves and everyone kind of spent that day doing their own thing.
The key to making it work for a large group like this is to sit down together, determine what everyone's expectations are, and decide how to make it work for you as a group. I will say I was scared to death about how it was going to work with all of us, but we had a fantastic time together and we would do it again in a heartbeat. Have a wonderful trip.