How do I change table in User Module?

I am trying to set up a small user group news forum. I have been able to tweek some of the settings, but I am a newbie at this and really want to change the Admin's User List view. Where the table has a column called "Member for", I would like it to show, instead, a new single line text field that I created in profiles called "Company name". How do I do this? Thanks in advance!