Megan,
If you can confirm that the TAs in question went to https://unc.zoom.us/ and logged in with their onyen/password, this will verify that they created an account. It’s possible they are in a different sub-account (e.g. belong to a department that has its own Zoom sub-account) that could be causing the problem – that could cause the error that you aren’t seeing them in “your” Zoom account. If that’s the case, we may need to consult with ITS to see if there’s a good solution.