Hi @chriswhiteley,
You can make use of the the WP_Query object to load all the posts ordered by a custom field in your case based on the last_name field:
The code will look something like this:
<?php
// query
$the_query = new WP_Query(array(
'post_type' => 'post_type_name',
'posts_per_page' => -1,
'meta_key' => 'last_name',
'orderby' => 'meta_value_num',
'order' => 'ASC'
));
?>