You can write your comment using the rich text editor. See a more general description of the editor here, including how to reference files, insert images and reference other stories. You can read more about comments in general here.
In comments you can mention users and teams by writing @ and start typing the name/username of the user/team. By mentioning a user you thereby require that the user confirms that he has read the comment – and he will be notified about your comment in Agemba and by email. Likewise, if you mention a team, either one team member or all team members (depending on the settings of that team) will be required to confirm having read the comment.