Cool! I know you'll have a blast. I'm a huge Disney fan, but just getting my first real exposure to Disney parks this year (went to Orlando for a week in January, Anaheim for 5 days next month). That said I'm a huge planner and really like forums like this one, so I did a ton of "research" on DISBoards.com. You can find all kinds of great information about both WDW and Universal there. Might be worth some time reading and asking questions if you have some specifics.
Some random thoughts I have off the top of my head -
IOA was great, but crazy with the crowds. We got there at opening and there was already a wait to get into the Harry Potter section of the park. The wait was 10 mins at the time, later in the day it got to be over 2 hours just to get into that section! Then another hour and a half for the headliner Forbidden Journey ride. So, my advice, go early if possible, and remember as a big group you don't always have to stick together. They areas are extremely well themed, but TINY. I'm not even sure your whole group could fit in some of the shops (like Olivanders). Another HP tip - Go through the regular line for the Forbidden Journey once. It's long, but worth it for the cool castle tour you get. After that, only use the single rider line. The ride is very dark and there are basically little walls between you and the other riders on the ride vehicles, so you really gain nothing but lost time by not taking advantage of the single rider line. When we were there the main line was hour and a half long, single rider took less than 10 minutes.
Disney - Fastpass is your best friend. :-) Once you get one you can get another at either the time printed on it OR two hours after when you got it, whichever is sooner. So, for the super popular rides like Star Tours, if you get a FP at 9am with a return time of 1pm, you can get another FP at 11am. The only caveat in that situation is that your second FP can't be for Star Tours (same ride), it has to be for different ride. In normal situations where you get a second FP at the return time printed on pass, your second FP can be for whatever ride you want.
With big groups, I think it can kind of depend on the group dynamic what you want to do. Some may want to all stick together and others may want to split up. If you want to stick together to do some major rides, there are some large vehicle rides where you could probably all fit in one (Pirates of the Caribbean, It's A Small World, Great Movie Ride). But for a good portion of the rides, the vehicles are much smaller, so you'll have to at least split up somewhat. You can also check out some of the theater like events as a big group (Philharmagic, Tiki Room, Laugh Floor). It just kind of depends on what your group wants to do.
Best times for the big headline rides without getting a FP is right at park opening, or just before close. Using FP wisely and riding the high capacity/less popular rides in the busiest times, my wife and I never waited more than 20 mins or so for a ride in January. It was awesome. Keep in mind the large vehicle rides/theater attractions like I mentioned are often very high capacity, so their waits will be considerably less than the headliners during busy times.
Ok, that's all I've thought of for right now. :-) I get very excited talking about Disney parks, they are such awesome theme parks. Maybe not big thrill ride wise, but their atmosphere (especially Magic Kingdom) is unmatched. Have fun!
