Only teachers and managers are able to promote certain students into teaching assistants (students are not able to do this). To see what kind of access your teaching assistant will get, please visit this article here, where we compare the functionalities of a teacher and a teaching assistant.
Firstly, make sure that your teaching assistant is already enrolled in your class as a student. You can invite them by sending them an access code. You can read more about it here.
Go to the tab Teacher- Users, and you will see the overview of teachers and students in your class. Find the user that you wish to upgrade, and click on "student" next to their name. Click on "teaching assistant" to give them this title.
Once you have done this, you will see the new teaching asisstant under "Teachers".