We had 19 in our group a couple of years ago.
My best advice is to go with the flow and to not try to keep everyone together all the time.
Plan some events that everyone will attend (a few nice meals perhaps), but past that, let folks follow their own muse. Touring with a large group, especially a diverse one is really difficult. I would guess you're going to have some thrill ride enthusiasts and some who will want to pass on them. With a little bit of planning an flexibility, everyone should be able to see and do what they want to do and still enjoy everyone else's company.
When our group went, we saw the Hoop De Doo Review and ate at the Rose & Crown and viewed Illuminations from the veranda together. We also spent the day together at Typhoon Lagoon. We had an adult night out at Pleasure Island. We stayed at Fort Wilderness in the cabins there so we also had a cookout there with everyone.
By the way, you are going at a great time of the year. The parks and resorts are decorated beautifully. Be aware that the parks do close early then. That should give you plenty of opportunities to spend tiime together with your group mates!
Most days, but not all, we all went to the same park. If your do plan to split up, transportation and adult supervision have to be considered.
Have a GREAT trip!