To mention someone in a comment, use the ‘@’ symbol followed by their username. When you mention someone, a global notification is created for them for your comment (see: Discussion Notifications). Note that marking a claim generates a global notification for the author by default, and so it is not necessary to additionally comment (see: Marking a Claim for Review).
If you are replying to someone’s comment, you can click the ‘reply’ icon, by opening the action menu on the comment you’re replying to (click the three vertical dots under the comment). This will pre-mention the user whose comment you’re replying to, as well as any other users mentioned in their comment.