To Check-In attendees at your event, go to Contacts. (Search for the name of the attendee if you do not see it in your currently open list.)
Click on Check-In button in the extreme right, and attendee shall be checked in.
The concerned attendee also receives an automated email that he/she has been checked in.
You can also check-in attendees using an iPad by logging in to the Dryfta plaform.