This article explains how you would change a user's system status from Employee/Manager to an admin. This will make the user a full system administrator.
If you would are looking to give them limited access to some areas (instead of full admin access), please go see the article linked below on editing access rights:
To give a user full admin status:
Click the employees tab
Locate and click on the employee you with to make and admin
Select 'Give Admin Status' on the right hand side:
Customer Services Team.