When you add a comment, and type @, it will let you select a user's name to autocomplete. Then when you submit the comment, that user will receive an email notification that their name has been mentioned.
BUT! There are some cases where this doesn't happen, and these are crucial to get funtioning.
1) If you REPLY to an existing comment and you type @, you DO NOT get the auto-complete option and you must manually type the user's name. The user will NOT receive an email notification about this mention.
2) If you EDIT an existing comment to add a user, you DO NOT get the auto-complete option and you must manually type the user's name. The user will NOT receive an email notification about this mention.