Hi - we just got back with a group of 15 back in January. We had 10 adults and 5 kids. I was the planner and tour guide. In fact, several of my group didn't even realize that there were maps in the parks until the end of the trip because they just always followed me. Some tips:
1) I can't advise on where to stay in villas - but I can tell you that we had 5 rooms at POP and it worked out great. They got us all really close (within 6 rooms).
2) I made all ADR's...tried to get everyone's tastes into account and also made a couple of different reservations for my sis and her husband who don't have kids cuz I figured they'd need a break from the kid-like meals. For meals - no matter how I tried to appeal to everyone - the favorites mostly ended up as the buffets - since people could more cater to their own likes there.
3) Figure out how much togetherness everyone is going to want. In our situation, 13 of us were together 90% of the time - my sis and bil had some times when they were off on their own, but 13 is a BIG group. If you are mostly going to be together: remember that it takes a LOT longer to do things than it does with just your own family. Even going from Splash Mtn to IASW would take forever. To keep a group that size together means a lot of standing to the side and waiting for everyone. And no matter how you try to avoid it - there are bathroom breaks all the time! In your planning, be sure to take this into account.
4) For tips at meals - We had everyone bring cash. it was a huge pain to wait for cards for rooms, get the wait staff to take the proper amount of credits kid/adult per card, etc. By bringing cash and being able to set down $15 per family (or whatever was appropriate) - it made it much easier. And by this point we were always ready to leave.
5) Also, we were frequently seated separately at restaurants. This was OK with us, but be prepared to wait a long time if it is not OK with you.
6) This only happened once, but we had a rule that if some of us made it on a bus but others didn't - either because some were walking more quickly or due to reaching capacity - that the others would just meet us at the destination. This worked out well.
7) By the middle of the week - the two sets of grandparents were getting pretty tired out. So they started to meet us at the parks around 10:30 - 11:30 am. This worked out great. We were able to get to the parks and bring the kids on all the thrill rides they wanted. Then when the grandparents were there - we were ready for a few shows all together.
8) For our CS meals - I would always give two choices to everyone. For example - in DHS We were near Fairfax Fare and I gave an idea what was there, but then also pointed the way to Starring Rolls and gave ideas there. We all met in the middle outdoor tables to eat together.
9) Group pictures are a pain - take FOREVER to situate everyone. We did lots of candid shots and most posed ones with just the kids because it took too long to get the whole group setup. But we got a few.
Best tip of all is to try to go with the flow. I am a super-planner and it took me a few days to be more easy going about things. It takes a LONG time to eat meals...it takes a LONG time to get from point A to point B. Most of our best times were when we were 'taking it easy' - like when I asked if we could wait a bit longer to be seated on the porch at Tony's and we got to watch the parade while eating dinner.
We had an AWESOME trip and I hope you do too!
