Customize wp_list_authors for user directory?

Hello,

I am trying to accomplish the following:

A
FirstName A LastName
FirstName A LastName
FirstName A LastName

B
FirstName B LastName
FirstName B LastName
FirstName B LastName

c
FirstName C LastName
FirstName C LastName
FirstName C LastName

And so on…

Basically use the functionality of the wp_list_author() but add the the letter of the alphabet to sort usernames by the first-initial in their last name.

Anyone know of this would be done? Perhaps a plug-in?

Thanks in advance!