Only Teachers, Authors and Managers are able to promote Students to 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.
In the Teacher environment, click on the "Users" tab.
You can use the dropdown menu at the top of this tab to select the different classes you teach.
Search for the name of the student, or find them on the list of students. Next to their name, you will see the tag "Student". Click on it to see the option "Teaching Assistant".
Once you select "Teaching Assistant", they will get this new tag instead:
A person can be a Student in one class, and a Teaching Assistant in another class.
You can revert them to "Student" by clicking on their "Teaching Assistant" tag and choosing "Student" instead.