If you're on the Super plan or above, you are able to invite assistants from the course participants page. Simply click the Invite participants button and select Invite assistants.

An assistant has limited access to your course. They cannot make any edits to content or settings. Assistants can only access data from and interact with learners that they have assigned. To assign learners to an assistant, find them in the participants list, click the action menu button and select Assign learners.

Select the learners assigned to the assistant in the menu. The assistant will now only have access to data from their assigned learners. That means the assistant will only be able to see completion data, submissions, reviews, discussion topics etc. from assigned learners.