Just like many of the other platforms you use - when you tag another user in your question or comment, the platform will notify them that they were mentioned in a post.  To tag a user, you simply start with the @ symbol, then begin typing their username. The platform will suggest matches, and you simply select the username of the person you would like to tag. As soon as you post your message, the user will receive a notification that they were mentioned, and a link to the post.