Hi @JoanneMueller,
If your school/institution allows you to add people on your own, the How do I add users to a course? - Instructure Community article should get you pointed in the right direction. If the +People button is missing for you, or doesn't let you select a TA role, you'll want to reach out to your local Canvas Admins, usually located in the IT / eLearning / Digital Education department, as they'll probably be able to add the person or advise you on any policies or procedures to do so.
Hope this helps!
-Chris
