You can format the text 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.
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.