Thanks to my colleague and campaign guru (and author of the great Obama London blog) Karin Robinson for these hints about using email to get people to attend events:
Most people, if they see [an email telling them about an event]... might think, "That sounds interesting, I might try to come along" but having made no explicit commitment they are highly likely to be "busy" when the moment comes.
However, if you ask them to e-mail you if they want to attend then 1) you can hint that there are limited spaces making it more attractive because a slightly rarer opportunity 2) you can then reply to them saying "Thanks for coming! I'll let the team know you're coming and we'll make sure to save you a place." - thus manufacturing a deeper sense of commitment and 3) You can then follow up with an e-mail just to the RSVPs reminding them that they've confirmed to come - people are more likely to attend an event if they get a personalised reminder.