We have also gone with groups that didn't really want to plan, in part because they weren't familiar with WDW. So, we sent everyone that was going a fun survey for them to complete by a certain date (don't give them much time). The survey was pretty comprehensive, and our guests thought it was hilarious. It turned out that almost everyone returned the survey, and we were able to take everyones input and plan the trip. If you want a copy, you can PM me.
As others mentioned, you can get ADR's for 10 people; we were willing to split into 2 groups, so that made it easier too.
Also, we find it easiest and most enjoyable for everyone to not stick together the entire time. We usually get to the parks together and ride a few headline rides together. Then we split into groups and meet again for lunch or something.
You can take a lot of time to plan, but if it bugs other people, they don't need to know what you are doing. Send out a tentative itinerary in July, and most likely, everyone will be impressed by the trip and excited to go.