Listboxes on userforms have change events. Right-click the listbox in design mode and go View Code. Select your listbox from the left-hand drop-down and Change from the right-hand one.
This will generate the skeleton code for your listbox's change event. Now you have to think of a way of checking passwords and write the code to do that.
Incidentally, are you aware that Environ("username") will return the username currently logged in? If it's suitable for your purposes, rather than ask the user to select his name from a listbox, you might consider just checking that instead. He would have had to get through the Windows login so he would have supplied a password at that point.