If you mention another member in a post in the following format @ExactUserName they will receive a notification*.  They will not receive a notification if they are mentioned without the above formatting or if the username  is shortened/abbreviated.

Typing their name in the above format will pull up an autofill field of usernames.  You can select from there (clicking on the name of the user you wish you mention), or type it yourself and if the link is correct a little window with their username/profile pic will pop-up.  

If you mention another user in a forum to which they don't have access (I.e. A mention of a poster with less than 75 posts  in AYTFJ) they will not receive a notification.

*If they have elected to receive  notifications for  this feature.