If you are an Admin of an Organization in Blackboard, you have the ability to add and remove users as needed. Here are some instructions along with an image containing the numbered steps. 

Adding a user:

Step 1 - Login to Blackboard, enter the Organization in which you are an Admin and select 'Users' in the left-hand menu

Step 2 - Click on Enroll User

Step 3 - Click on Search for User to Enroll. From the next screen you will enter the username of the person you wish to add as well as pick their role for the Organization. Finally, click "Enroll"

Removing a user:

Step 4 - Click on the checkbox in front of the user you wish to remove

Step 5 - Click on Remove Users from Organization