To mention someone in a post, just write an @ followed directly by the username of the person you want to mention. A drop-down list will appear with suggestions as you start writing the username, you can just click on one suggestion to select it.
When a user is mentioned in a post, he or she receives a notification by email. You can see all the posts in which you are mentioned on your user profile (click on the "Profile" menu on top of the page, then in "Profile Info" you can see select the "Mentions" option) If you want to disable the email notification when you are mentioned somewhere, just go to the "Mentions" option in your user settings, and in the bottom-right of the page you can uncheck "E-mail mention notifications".
Please don't abuse this feature, or it will be removed! Enjoy!