Group dining is your best bet, but your times will be limited as they mysteriously make a limited number of tables available. So, you'll want to jump at your first chance to make the ADR.
I've seen a table of 16 or so set at Narcoossee's, so you might have some luck at the small-ish restaurants. I think even with Group Dining they give you the disclaimer that you might not all sit together. With our group of 9, they always said that, but we were seated together each time.
Especially if you are celebrating something, I think the restaurants work hard to make it work out for you.
As mentioned above, Boma is a very easy choice for a large group. I'm sure Whispering Canyon could push together enough tables, and if everyone wanted to order the family-style skillets, they could probably get you served relatively quickly.
If you are going as a "Grand Gathering," there are a few options there for sit-down meals with entertainment. Spirit of Aloha and Hoop-Dee-Doo are other (pricey) dinner entertainment options.
Sorry -- ideas keep coming to me: I've also seen large groups at Chef Mickey's and Mexico (San Angel). Group Dining will have more suggestions: 407-WDW-DINE .