Quite often I will arrive at a users machine and need to perform an administrative task, to do this without logging them out I choose to use the Terminal and change to an administrative user that way, here is my guide on how to do this.
Open the Terminal application
(Located in /Applications/Utilities)
for this example my administrative user is named localadmin, so whenever you see this just replace it with your administrative user
type the following
when you hit enter you will be prompted for your password, once you enter this correctly you will now be running as the administrative user.
You will not be able to SU to an admin user account without a password set