Proposed by Vicki Tardif-Holland, http://lists.w3.org/Archives/Public/public-vocabs/2014Oct/0059.html
"Schema.org currently has an RsvpAction; however, it is not clear how to
send the actual RSVP response and the number of guests who will attend. To
that end, we propose adding the properties "rsvpResponse" and
See: http://sdo-actions.appspot.com/RsvpAction http://sdo-actions.appspot.com/RsvpResponseType"
Queued for final review and hopefully release. See http://sdo-venkman.appspot.com/docs/releases.html#sdo-venkman