Navigate to the WebFAC module at the bottom left of the main Blackboard page
Click on the Class Roster link
Use the drop-down box to select the semester you need. It should start with the current semester selected.
Click the BB Management tab, which appears in blue under the green banner.
Click the blue icon under the Guests column next to the course to which you would like to add guests. A section to manage guests for this course will appear below the list of your courses. You may need to scroll down to see it.
Enter the WUSTL email address for the person you would like to add, select Guest Student or Guest Faculty from the drop-down box, and click the add button. Guest Student is the role I recommend for everyone added.