As a general rule, you should work from the student work rather than the roster to ensure you grade all work you receive.
If a student is not on a roster, you should add them if you can. If you can't add students to the roster, it is because your roster is imported from Google Classroom. You'll need to message the teacher to let them know if there are students you're unable to grade because they are not on the Google Classroom roster.
If you're missing work for a student on the system roster, you can skip the student.
If there are significant discrepancies between the roster and the work you receive, you should contact us to make sure you've received the right work.