HelpDesk allows users to search and mention other JIRA-users in comments by typing "@" and user name after it. Search and mention works only with JIRA-users.
The user who is mentioning user must have a global Browse users permission.
The mentioned user will receive a notification with link to issue in JIRA. It is not recommended to mention HD-users, because of JIRA link, so the user could not enter the issue directly.