A JIRA user is by definition any user account in the JIRA system with the JIRA users permission, i.e. anyone who can view the site while logged in. The permission to login is configured in the Global permissions. As the JIRA administrator you can grant other project permissions to any users in the system (create, edit, etc).
Unlimited anonymous users are permitted on all licenses. This is done by granting the view permission to 'Anyone'. You can also allow users to create issues anonymously.
If you need JIRA to recognize a person in any way - to send outgoing email notifications, to allow them to view particular issues that are not open to 'anyone' (the general public), then they will need to be a user in JIRA.
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