So, to start, just press the @ button on your keyboard; I have a Microsoft, and all you have to do is press Shift and at the same time the key with the @ symbol on it. Then, type an email into it. Usually, it will look like this @email@example.com. But sometimes it will look like this @brekteacher. It will highlight in blue when you are done, or when you type in a recognizable email address. Then, you can click on it, or the other people on your team can, and in a minute contact the person whose email was put in an @mention.
(The person recieves an email when their email was used in an @mention.)