How do I assign User Administrator access to a user?

Select the "User Management" tab located in the left navigation menu and then "User Accounts" from the dropdown.
Locate and select the user you would like to allocate the User Administrator privileges too.
In their User Details page, scroll to the bottom and click the slider toggle on "User Administrator". Once enabled it should look like the below.
Select the user group you want the user to have administrative rights over, you can select multiple options in this menu.
Finally, select the "Update" button.

After reading this article, you will know how to assign User Administrator access to users.