Blackboard, in an effort to make the user experience as difficult and incongruous as possible, has made adding a new instructor fittingly unintuitive. First, you'll need to enroll the new instructor as a student. Once you've done that, click on "List/Modify Users" in the "User Management" pane of the Control Panel and search for the new instructor. Click on the "Properties" button next to their name. At the bottom, you can select their role in the course; change it to "Instructor" and click "Submit."