1) Terminal window

     sudo adduser name_of_user admin


2) Graphical Interface

Go to System--> Administration --> Users and Groups.


Then:


(1) Click on the name of the user that you want to elevate.

(2) Click on the properties button to open a new settings window.

(3) In the new window, click on the User Priviledges tab.

(4) Check the box next to "Executing system administration tasks".


Click OK all the way out, and that user should now be able to sudo.