Click here to know more about creating a template in Salesforce Classic and here for creating a Template in Salesforce Lightning.

Note: Please choose template type as "Custom Template". Salesforce supports only custom template for adding the Contact  QR codes

Enter all the required information in the email template for inviting the Contacts for an Event/Shift/Job. Make sure you add the Contact QR Code string in the email template. The Contact QR code is shared in the email only if the Contact QR code string (as below in Yellow) is included in the email template.

Contact QR Code string: "<img src="{!Contact.Id}" alt="Logo"/><br><be>"

Once the templates are created in Salesforce use those templates and mass email to Contacts. The Contacts can use the QR code from their email for Check-In and Check-outs.

Click here to know more about sending mass emails in Salesforce Classic and here for sending mass emails from Salesforce Lightning