I've gone to WDW twice with big groups, the first time we had 16, this past Sept. we had 22. Here are some of the places we ate:
LeCellier
HDDR
LTT
50's PT
Mama Melrose's
'Ohana
WCC (we actually did this for my sister's wedding dinner also, with about 40 people or so, and it went really smoothly)
Chef Mickey's
You may have to take either the first or last seating of whatever meal you're wanting to book, we had to do that most times because that was the only time they could fit us all in. Also, with a group over 10 you have to call group dining, their number is 407-939-7707. Hope that helps, and good luck!