Is it possible to contact an external person via mail and ask them to comment on an issue in Jira without them having a user account.
Is this JSM? Assuming so, the external person would be a customer and that customer must be registered as a user unless you open up the project to be public. In any event the individual will be created as a customer once they engage on an issue. If you could explain your used case a bit more I may have further information to share.