I was once in a guild that had a site that sent automatic email reminders to people who had signed up for an event. Each user could set the amount of time he or she wanted to be reminded in advance in their account settings. Thus, a member could set the email to be sent 12 hours in advance, three hours, etc.

We found this to be VERY useful at limiting the number of people who forgot to show up for an event they signed up for. This is a feature I'd like to see here as well.