Good morning, @LisaJMCGRATH ...
The terminology that @saradean is using in her reply is not exactly correct in terms of Canvas (though she has the right idea). Here are a couple Guides that should help you manually add/remove people in a Canvas course.
Since you said you are an administrator, you should be able to add/remove users from the course in question. Instructors may not be able to add/remove users in a course based on permissions that the Canvas administrator has set at the Canvas account level.
I hope this extra bit of information will be helpful to you! Sing out if you have any questions...thanks!