Hey Martin,
There’s unfortunately no way to do this “out-of-the-box” with the plugin – it is technically possible with some extensive code customization, however.
One thing worth noting is also an additional, “Un-official” plugin made by one of our awesome developers, Barry. It’s called “Event Rocket”, and is free and also unsupported. But it does have some basic RSVP functionality that many people have found useful. Check that out here! → https://wordpress.org/plugins/event-rocket/
I hope that helps 🙂 And P.S. – we are going to be adding RSVP functionality to our plugins soon. I genuinely don’t know what the ETA is on that, but we’re hoping to include it before 2016.
Cheers!
George