Just remember that 80% of the people who will claim they are going to show up won't.
You want to be sure she is 100% up for it and won't change her mind at the last minute. You also want to see if she can get any female friends to be there even if not participating so it won't be a total sausage fest and the guys will at least have someone to talk to while not participating.
You need to get a suitable location. If it's a hotel room make sure it's big enough to accommodate the max number of people who all might be there at once. Also make sure you have enough drinks (at least bottled water), lube, condoms, etc.