A course master or instructor with edit-roster privileges can edit the in-gradebook status of students.
Click the "Edit" link in the "In Gradebook" column
Hover over the checkbox in the In Gradebook column to see the Edit link. Clicking this link will make the checkbox active so that you can change the status. When this box is checked, that student will appear in the Gradebook.
Note: If the link does not appear, then your current enrollment does not have sufficient privileges to edit the in-grade book status.