Hey Vendredi,
The best way to do this would be to send an email out to the invitees with a link to a webpage with a Marketo form on it. This web page could be a Marketo Landing Page or an external web page with a Marketo form embeded on it.
To get a list of RSVPs, you would then create a Smart List using the "Filled out Form" filter. On that filter, you can specify the form the leads filled out to RSVP. You could also create a custom view for this Smart List to view only the data that you want to see. Instructions to do so can be found here: https://community.marketo.com/MarketoArticle?id=kA050000000L6YkCAK)