Last weekend, I attended a conference (Singularity Summit) at Stanford which was free. They had a large hall ready to hold 1800, but they got enough registrations to put around 700 on the wait list. However, at the actual event there were a few hundred empty seats in the balcony.

This is because when something is free, people register to go "just in case" and as many as half of them will not show up. In fact, it's often suggested that it's better to charge a nominal fee that nobody will be scared of, because it can actually make more people attend, because they said they would and paid to.

(The Long Now foundation, when it decides an event is going to overflow, institutes a $5 "reservation fee" to assure a seat. Normally their events are free with $10 suggested donation. Before they did this I had the disappointment of driving up all the way to the city to be turned away.)

Everybody is so connected now that it seems a just-in-time confirmation system might make sense. Done primarily on the web, those who wish to go would have to confirm via the web or via automated phone system the day before the event. Late that evening, or even the next morning, waitlisters would be mailed and given the opportunity to confirm. In addition, a phone-in number could provide the stats for those driving to the event, to know if it's worth going.

Airlines are doing OK allowing at-home check in 24 hours before the flight, so this can work too. Even with this, some would confirm and not arrive, and that percentage could be learned to do the right overbooking.

Attendees who confirm would be given a page to print, which would tell them the confirmation procedure, and give the URLs and phone numbers for confirmation and stats. This page would in turn be their ticket. No need for fancy bar codes on it, for a free event you aren't going to get people copying them or forging them.

In addition, for longer events, people leave and don't return, so the online and phone status reports could indicate that while the event had been full, it's now safe to come. Truth is, you are still going to get enough no-shows that those who didn't confirm can probably still show up, but this way they can be sure.