To RSVP to a reunion you've been invited to, go to your Reunion Center, click the reunion event, then click RSVP now. You'll be able to select your response, add optional comments, and include your number of guests.
To edit your RSVP any time prior to the event, return to the event page and click Change RSVP. Once the reunion event has started, the details are frozen and you would no longer be able to make changes to your RSVP.