If you've been given access to a student's portfolio, you can provide requested and unrequested feedback on the items that have been shared with you. You can give feedback in the role of Teacher or Coach (or Student). Before giving feedback, make sure that you set your role to fit the role in which you want to give feedback. You can change your role on the homepage of your own portfolio.
In Drieam's manual Request and provide feedback, you can read how to respond to a feedback request, how to post unrequested feedback, and how to modify or delete your feedback.