For chek in, you'll need a QR code scanner app. You can use any QR scanner that recognizes and automatically opens a URL. We recommend 'QR Scanner - Fast Scan' by Sabrina Holzer for iPhone and 'QR Scanner' by QR Scan Team for Android.
You can add an entry for each participant at: Participants -> Action -> Check In-> Comment for Check in. The comment will be displayed when scanning the ticket.