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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.