The way I costed Japan was a little different to this trip. I just picked an itinerary for Ian and I and made the trip open-invite. I guessed at how much everything would cost, added 20% and that was the deal. It would be pretty hard for a non-Japanese speaker to get around and do everything we did.
For Antigua, since people speak English it's going to be way easier for folk to make their own fun. However are there any group activities you have in mind?
Lets assume 7-8 people will show up and rent a house for 8-10 days. What's the approximate cost for that and when do you need to book them by? We're going to have to rent a car or two also.
Basically, by the deadline for booking the villa, you're going to need confirmation and deposits from all who are staying. I would make it a fixed length and cost (say 9 nights @ $xyz) for everyone and ask people to put up or plan on finding their own place to stay.

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Loaded-Gun.com - I don't know what the hell they are talking about or why they are even there. They don't make serious points and they don't joke, but they still manage to make a lot of posts somehow.