Summary

According to our documentation here:

If you create or respond to a request via email, add a request participant's email address to the TO and CC fields. The participant must have a JIRA Service Desk account to be added to the request.

However, the last sentence is incorrect(outdated) since January 2016. Please refer to the comment here:

The solution that has been implemented here, is as discussed above:

• For a public signup enabled service desk
• When processing the email for create / comment, it will check all TO / CC addresses and if they do not exist, new customers will be created and invited to the service desk, and added to the issue as Request Participants.
• In addition to this functionality through email, in the customer portal request details, if allowed by permissions, customers can also invite new users by entering their email address
• both of these are only with public signup, and the request security is configured to allow customers to manage participants