Find out from the machine name, what user is logged in

thank you. Is there a way to find out from the machine name, what user is logged in or who the machine belongs to?

I can actually find out from the username what machine they have because there is a field called the wWWHomePage that gets populated with the computer name

how do I do that? 

Also I have a list of email addresses. How do I find the username from the list of email addresses ?

thanks