In the majority of the cases, searching a JIRA user with the display name, username or email address brings up the relevant results. But if you cannot find a JIRA user while adding him to UpRaise in company directory, try following actions:
If the user is JIRA administrator, try searching the user with 'admin', 'administrator'
Check if turning on/off limited roll-out setting has an impact on your search result
Make sure that you are searching in the right section by, click on Search users link to get search options.