How do you create an RSVP form?

If this is for a list of existing leads from your DB you want to invite to the event, then in your email you could just have a link. The text in your email to say just click here to RSVP. Then they are taken to a thank you page. In your program, you're listening to that click from you're existing lead to update a status and send out the confirmation email.

If this is to promote on your website then you will need a LP with a form to capture their details (could just be an email address) then again, your smart campaign listens to this form to trigger the confirmation email.