You can tag another user in a forum post, a profile post, or a profile post comment.
To do this, type an @ followed by their user name.
I got great help from @Mr. KnowItAll and @SueFromDenver.
If the user you mentioned has set their preferences to receive notifications when they are mentioned, they will learn about your post or comment.
Receiving notifications when you are tagged
To receive notifications when someone else mentions you from now on, go to your Preferences page and adjust the settings for
Receive a notification when someone… > Mentions you in a message
Receive a notification when someone… > Mentions you in a profile post or comment