As a general rule, do not manually add students to matriculated courses. Student registration and enrollment must be handled by GET. Canvas receives changes from GET multiple times per day, with students experiencing minimal delays due to synchronization. Students may encounter a delay in enrollment when waiting on a permit or for a hold to clear on their account. These issues must first resolve before they can be enrolled in Canvas. Manually adding students bypasses this process. Students might falsely assume they are officially enrolled, when they are not.
In the process of adding a TA or GA to your course, all you need to focus on the role assigned to them when you click on the Add People button in the People area of your course. The role of a Student Grading Assistant in Canvas is called TA. They will have limited access to edit content, but may assist with grading assignments.