Depending on what permissions you want to grant the instructor there are several different options:

• Primary instructors, secondary instructors and teaching assistants share all of the same privileges. Authorizing another instructor to one of these levels will allow them to do things like create and publish assignments, change dates and grades and more.
• An importer is unable to make any changes to your course but has the ability to view your assignments and copy those assignments into one of their own classes.
• A limited TA is able to view and change grades but is unable to manage assignments in your class.