Students are not guests in your Course Tools / Blackboard course. They are students. Adding a student who is not officially registered in your course as a guest does not allow the student to do anything but view documents and areas of your course that you have made public to the world. Guests can’t complete assignments, engage in discussions or take a test.
Please contact Student Central or Student Enrolment Services for verification of a student claiming to be enrolled. If they are a student, then contact the help desk to have the Blackboard Administrators add them as a student, not a guest.