Events pages allow you to track RSVPs and attendees, since not everyone who RSVPs for an event is actually going to show up. 

Once you are in your event page, go to Event Details > RSVPs. This is where you can track all of the RSVPs for your event. If the event is connected to a Facebook, or Meetup event, those RSVPs will appear here too. You can manually add new RSVPs by clicking on the new RSVP tab. You can also download and print this list. When you have an event, it's a good idea for you or the host of the event to have this list of RSVPs handy in order to keep track of who attended. 

If you go to Event Details > Attendees you will see the same list of RSVPs you saw in the RSVPs tab, the only difference is these have check boxes next to them. Check only the people who attended the event and then click on Update Attendees. These people will now be tagged as having attended and will be moved to the attendees list.