I just planned a trip for 20 family members at Disney in December. We had a great time, but mostly because we laid the groundwork ahead of time. Being the experts DH and I (pretty much just me, really) planned everything. We know the family well and that everyone has their own speed, likes, dislikes, and maintenance levels, so we did our best to anticipate those.
For instance--took 7 smaller parts of the family. 4 of those are VERY picky about everything....so I talked up the advantages of the Deluxe resorts and we stayed at AKL. There would have been complaints at moderates (outside entrances, no bell service, distance between buildings/food court/bus stops/etc) so by staying at AKL, we cut those complaints before it started.
I knew they'd constantly ask us for advice and tips and directions, so I made up a welcome book that had basics of everything, suggestions on where to eat, how to get to all our planned events, and a calendar of our events so they couldn't forget.
I put out from the beginning that since it's a vacation, and everyone is supposed to have fun, that everyone would be free to do their own thing so they'd be guaranteed to have fun!! We planned one group event each day (mostly dinners) and I made them opt in for it...they had to email me a "yes" that they wanted to do it before I called to make PS....but they were free not to do it, even though it was a whole-family event. For instance, one couple had friends in Tampa they hadn't seen in years, so they skipped one dinner to drive to Tampa to see their friends. No big deal....we saw them all the other 5 nights!!
We got an extra room at our resort that we used as a hospitality suite, that we put in a fully stocked fridge (beer, sodas, milk and juice for the kids), tons of snacks, decorations (it was Christmas!), and copies of all our Disney info and planning books. It was important to us to have a place to gather, without imposing on anyone in their room. We met there every evening to recount our day's adventures and see who was going where the next day. If folks were interested in the same park, then they went together!
The only hitch was my mom (the rest of the family was DH's). She couldn't get with the program. In spite of agreeing to meet at the elevator or hospitality room at a certain time to leave for the park, she was never there and had to be fetched. (She stayed with DH and our kids since the rest was DH's family and she wanted to spend time with the grandkids. It might have been different if she had someone else to walk around with, but she's single, so we were it.) At the parks she only took smoke breaks when we were trying to get somewhere, rather than when she was waiting for us to ride something (she's not much of a rider). Just didn't think ahead and consider others.....
Overall, the trip was a huge success. Since everyone DID have a great time (they got to do what they wanted...not get hauled around everywhere by me who did all the planning) they now have me planning a summer trip for all of DH's family at the beach. Let's just hope I can make it as magical...I like these free vacations!!
Good luck! A large family trip can be fun, as long as you're realistic. There's no way to have 10-20 people up, dressed, and ready to walk out the door at the same time....in a lot of families it's too much to ask to have 2 people ready at the same time!! For some it won't be a vacation if they have to wake before noon, but for others it won't be fun unless they're at the parks open to close. Navigating a park with a crowd like that is VERY difficult and trying, and when you add in different ages and interests, it's too much to expect everyone to stay together the whole time. Make everyone happy! Plan some opportunities for family togetherness (meals, events, etc), some for small groups (backstage tours, golf, cruise), and leave time for the individual families going to plan their own thing.
Have a great time!
Katie