Here`s an example: Not so long ago, I was part of an online group that
came out of AOL and eventually formed as an email group. We all were
part of that first bloom of online friendships, dating, meeting, and so
forth. There were about 50 of us, all around the same age (Baby
Boomers), and we inevitably decided to meet for physical parties.
Although we never intended for things to get too formal, there were a
lot of logistics involved, and so a couple of folks who lived in the
town we picked would act as "organizers." They weren`t really event
planners, more just helping out with the details.
We`d pick some town, either because it was in the middle of the
country, or because it had some interest point we all had in common. So
one of them was in St. Louis, although nobody actually lived in the
city.
One or two of us would volunteer, and we`d set up two basic things that
sound similar to your situation. The first was hotel space, the second
became a sort of traditional Saturday night dinner. We`d arrive around
Friday, leave Sunday morning, and everyone was responsible for their
own plane tickets and rooms, dinner, and so forth.
Additionally, since we`d take a hospitality suite or room for 2 nights,
with adult beverages involved, there`d be ancillary costs plust liquor
bills.
Our time would usually be at least a couple of months out, what with
organizing 50 nut jobs to do something in unity. With a weekend in
mind, the "facilitator" would call around and ask about group rates
from various hotels. They were always ready to talk, and would usually
have a minimum number of rooms to start. From there, we could usually
negotiate some sort of deal that was attractive.
Same with restaurants. We`d call around, find a nice place that could
handle a rowdy bunch, often 25 people, but once in awhile up to all 50.
Either they`d have a banquet section, or they`d have room to set us up
in a whole section. Only once did we take over the whole restaurant.
That too was negotiated, usually by choosing only a couple of options
for the dinner menu.
In some cases, we just booked the space, maybe with a slight discount
on drinks, and we`d all pay for our own meal on a massive group tab.
What happened each time was that the overage on the bar tab, along with
various unforeseen incidentals led to a shortfall. So before everyone
left, or sometimes later, by email conversation, we`d have donations.
People kicked in whatever they could afford to make up the
difference...usually around $300 or so.
Out of all this, we never had any bar, restaurant owner, or hotel chain
even question negotiating. Having a large group is so common in the
entertainment industry that it`s assumed. That being said, we also all
just assumed we`d pay some sort of fee, whether as a deposit, or
booking our rooms in advance, or whatever.
So it comes down to charging your event-goers a fee. As Robert said,
when there`s actual money involved, people get a whole lot more serious
and committed.



