Since Elgg 5 this feature is available by default.

Nice, I have test in my local computer -- but user has mentioned to the discussion never receive notification?

This will work under the following conditions:

• cron must be running on the server
• the mentioned user must enable notification delivery on "Notification settings" page
• "Site Notifications" plugin must be enabled to deliver site notifications
• the mail service must be configurating to deliver notifications by email

thank you, now notification is working!