@VLH ...
The way I understand it...if a Canvas administrator is adding users to a Canvas environment, they are added at the account level. Once a user is added at the account level, then at the course level, that user can be added to the course with a particular role such as "Teacher" or "Student". Since people are not given a role at the account level, this means that a user can be assigned the role of a "Teacher" in one course and a "Student" in another course.
It is also possible for users to be enrolled in courses via SIS (Student Information System) import. This is also a task that can be done via a Canvas administrator. Instructions on how to do this can be found at:
If you are a Canvas administrator, you might want to take a look at the other "SIS Imports" Guides to see if they would be of help to you. You can find those located at:
Hopefully these resources will be of some help to you. Sing out if you have any questions...thanks!