Including Participant ID in Event Registration Email
Susan, Welcome to CiviCRM stack exchange. Its possible to add Participant ID in email sent when a user registers online. You need to update the message template to include the {$participantID} token which prints the participant ID when rendered. 

To update the message template navigate to CiviCRM >> Mailings >> Message Templates. Locate 'Events - Registration Confirmation and Receipt (on-line)' under 'System Workflow Messages'. Click Edit. Modify the message template to include the {$participantID} token either in subject, html or text field.

HTH

Pradeep