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 the user must have global Browse users permission.
The mentioned user will receive a notification with a 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.
Use the Invitation functionality to add participants to the issue and to allow them to get notifications by your comments.