Every year I take charge of my gf's birthday, ski trip and usually we have 15-20 people. The easiest way to manage that many people is TO TAKE CHARGE.
I set dates, times, and inform the group of all the plans... and if they don't like it, they don't have to come. Getting money from people is usually the hardest part (early enough to make a deposit on a room), so I usually get the money AT LEAST 2-3 months in advanced. If someone backs out, too bad. They can either find someone to take their place, or deal with it.
My advice is to set a date that YOU want to go... and let the others follow. Make sure to get the money from anyone interested asap so you can book a room. Also, plan who should get what groceries/alcohol ahead of time and divide it equally so everyone is paying roughly the same amount.