If you would like to let a user know you have mentioned them in a post. When you type their user name put the @ sign in front of the name like this @jim. When you have typed three letters of the name the system will drop down some probable names. You can keep typing or click on a name if you see it.
When you click on Post Reply the @ will disappear, the name will be bold and the person will get an alert to let them know you have mentioned them.
