When someone creates an event and invites you to attend, you will get an email with a link to the event, like the one shown below:
Click on the RSVP link to see more information about the event and to RSVP. The page will look like this:
All you have to do to RSVP is choose whether you are coming and click RSVP.
You can also RSVP for an event by clicking on the event from the calendar. If you have the "manage_events" privilege however you are going to see an event form first. To get to the RSVP form, you just need to click "yes" or "no" under "Are You Going?" as shown below: