First, I want to apologize for your question sitting in the community for so long without a response.
Second, it looks like you have stumped the Canvas Community. Were you able to find an answer to your question? I am going to go ahead and mark this question as answered because there hasn't been any more activity in a while so I assume that you have the information that you need. If you still have a question about this or if you have information that you would like to share with the community, by all means, please do come back and leave a comment.
No, unfortunately, I did not find an answer. It does not seem possible to add teaching assistents to their student groups, which is pretty unfortunate.