I have events on my website https://jeena.net/events/ plain HTML marked up with h-event http://microformats.org/wiki/h-event and I can receive RSVPs with help of https://www.w3.org/TR/webmention/

It was fairly easy to implement, the only problem is that not many of the people I know have software which lets them RSVP too, therefor I also connected it to Facebook.