Live Attendance is the easiest way to keep track of the attendance at your event, but you can still manually record attendance after your event is over. Here's how:
Click Attendance in the dark blue bar.
Check off the names of the people who attended
Click Record Attendance when you are finished.
You'll be able to find those who attended later by using Search and changing the "Registration Status is not Cancelled" options to "Registration Status is Attended."
You can also email those people from Send Email and choose People Attended.