Authors divided alphabetically

I'd like to create a page on my site listing all of the site's contributors with info like their bio, URL, etc..

I found this code which is perfectly suited for my needs. However, what I'd like to do is have this code break contributors up alphabetically by last name (or by username if a full name is unavailable) with a header and anchor tag for each letter. Ie.:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Bill Abrams
Tom Anderson

B

John Biskup
Steve Botwin

C

etc.

Any thoughts on how to modify the linked code above to achieve that?