Instructors and students are enrolled in Blackboard courses by means of an import from the student information system. If the Registrar shows you as the instructor, or as a student enrollment, that enrollment will be reflected in the Blackboard component of the course. However, you can manually add other users to your course, and edit or define their role in the course (student, TA, Course Builder). See the Manage Users link for instructions on that, as well as definitions of the course roles.
You can also create groups within the course to provide collaborative spaces, or to define release of materials to subsets of students.