Reopened user is the user who change issue's resolution field from a value to EMPTY.
To create a First/Last Resolution user custom field
- Navigate to JIRA Admin > Add-ons > Enhancer Plugin Menu > Custom Field Configuration and switch to User tab.
- Click on button.
- Fill the dialog
- Pick JEP - First Reopened User or JEP - Last Reopened User in custom field drop down.
- Give a name to the custom field
- And click Add Custom Field button. That's it.
This custom field can be used to sort or search in issue navigator.