Mention a GitHub user in a comment:
In your browser, navigate to the applicable repo on GitHub.
Create your comment or issue, as described previously, and then in the comment text box, type @, followed by the user or team name, within the comment.
Note: When you type the @ symbol, a list appears that contains GitHub users who are collaborators on the applicable project and anyone who is participating in the project’s comments. The list uses autocomplete as you type, so that you can filter the list easily.