I've been using this module happily for a couple months now, thanks to New Zeal. Just wanted people to know you can add signup functionality through a workflow-ng configuration listed here: http://drupal.org/node/277721

What it actually does is automatically sign people up for all nodes that have the signup option enabled, and also suppresses the signup form, so users don't see or do anything extra, yet you still have a nice singup registration list for all your events =)

PS. In order to suppress the signup form from the user's end, the above link has you create a module called my_tweaks. Using Drupal 6.4, I had to make a modification to it to get it to work (and also to get it to supress the signup form on both event nodes AND product nodes, since I wanted to track signups for both). Here is what I have:

<?php
function my_tweaks_signup_suppress($node) {
if($node->type == 'product') {
return TRUE;
}
if($node->type == 'event') {
return TRUE;
}
}