You can search for attendees, and check them in (admit them to the event) from the admin dashboard. A common use for this function is to search for the check-in status of attendees who have forgotten to bring their tickets.
Search for the name or email address of the person you want to check-in, and then click "Not Checked-in" next to the relevant person. The status will be changed to "Checked-in".
The number of people checked in is displayed in the box next to "Checked-in". To find who and at what time someone attended the event, after the event has finished, you can download a full check-in history using the "Download Check-in List" link in the left hand navigation.