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.
To mention helpdesk-users in issue the "See Invite button" permission should be enabled in HelpDesk administration menu. This feature allows users to input user's e-mail to sent an invite to the HelpDesk issue and if this user has permission to view this issue, he/she cam add themselves to issue watchers.