We've done large groups before and have 15 for December.
i HIGHLY recommend
Dreams Unlimited Travel- there are links all over the site- one at the top here. Pete, who runs these boards, is a part owner, so you know you are getting good service and knowledge. They will look for all current discounts for you and apply them when they come up. They will even do an itenerary for you and book your ADRs. I've used them before and always been extremely pleased. They focus on disney only, so they are experts. We have our Dec trip booked through them and I just did the DIS podcast cruise through them as well and it was great.
they send notices and updates to you and you can log in to their system and view your reservation and itenerary. We prefer to create our own (way too detailed) itenerary, but we really like having help on booking the ADRs, so we don't have to sit on hold.
Our plan is too always HAVE A PLAN. we make sure everyone has copies of the itenerary. We don't plan it out ride by ride, but we do plan which park each am and pm, as well as which restaurants (we do
DDP) and parades/fireworks. We usually do plan the first few rides, since we usually hit e-tickets ones and do FPs early. This way everyone knows where to be and if they don't want to follow the plan- they just do their own thing. It helps with NOT wasting time trying to agree on something once you are there. We also plan to be flexible. If we need to adjust, we do. And again, be prepared to split up, especially for dining- the restaurants simply cna't seat 15 people together everytime, heck most of the time they can't and you'll be split some.