Run As shortcut username fill-in

Gurus,

You know how you can click Run As and enter alternate credentials in which
to run a program? Well when you select the "The following User" radio
button, and select the drop-down menu, sometimes their are usernames
pre-filled in for you. What is the criteria for the names already populated
in this list, and is there a way to add another name so I don't have to type
it in each time?

--
Spin