We have gone with as many as 15. It can get a bit hectic, but I think everyone had a great time.
As far as advice....
Plan ahead what everyone plans to do for meals. If you plan to eat together make sure everyone has the itinerary and knows how early to show for a meal.
Don't try to all stay together, you'll end up wasting a lot of time. What we did was split into 2 parks a day. I had an itinerary done for 2 parks each day, and on 2 days I made it for 3. I passed out all the itineraries and let people pick and choose where they wanted to go that day. And of course no one had to choose to do any of it, it was just a guide for those that didn't know how to space things. What i didn't include was touring plans. We had way too many people for that. I simply put where and what time for breakfast, the time we planned to be at the gate, an approximate time for a counter for lunch and then a hard reservation for dinner. Plus things like shows or fireworks.
It worked out well. We all ate dinner together at night and ended up watching fireworks together, but we didn't try to stay together at the parks. Mostly because everyone had a different thing that they wanted from the trip. Yet everyone wanted to see each other at least once a day. So since everyone had the same itineraries on them we all knew where the others were, approximately.
Another thing I can think of is no one should be made to feel bad if they don't want to do something. The first time we went as a large group there were some people that just didn't have the go go go that most of us had. Everytime these people wanted to leave the parks they were made to feel like they were wrong in some way. So they would end up staying, and of course being miserable. So the nexttime i made the rule that people are responsable for themselves, if you want to go off no one is to make you feel bad AND you are not to expect anyone to follow you. That was also an issue, when some people wanted to leave they expect everyone to leave and would get mad if they didn't. With that rule we avoided arguements the next time around.