Creating a front-end form by plugin

Hello,

I’ve been looking for a way to do this for several days now, but to no avail: On one page I have generated a dynamic list of events which the admin can create in the backend. This works wonderfully and the content is added to the page using the Shortcode API. The admin just has to create a page with my [eventlist]-shortcode. So far so good. For now let us say the user has created this page at:
http://www.mydomain.com/my-event-list

Now I want the visitors to be able to sign up for these dynamic events, but I can’t find a way to display the form in the front-end. I would like to load the signup-form when the user goes to:
http://www.mydomain.com/my-event-list/signup/title-of-event
http://www.mydomain.com/my-event-list/signup/another-event

But of course this throws a 404-error as I have not registered any actions yet.

I’ve looked at:
http://stackoverflow.com/questions/13303079/add-a-custom-page-with-form-in-wordpress-using-plugin-for-frontend-user

But neither of the options seem to be feasible as there are multiple parts in url. So how do I replace the 404 error with a page of mine, and how can I do this when the url is right? I don’t want to hardcode any other part of the url than /signup/ as the system should work in any wordpress installation.

Best regards
Morten