You can mention other users using the @ symbol in the comments.

You can use it by either using the displayed name or the user id:

e.g.

@Peyo

@1

you can find the display name either on the profile page

or you can click on their name in a comment and the name will be auto added to the comment:

you can find the user id in the url

after a succesful mention the name is replaced by a special markdown: