Profile: kaiD-2

Problem:
Add custom user roles to author quick edit dropdown.

Solution:

This can be done by adding the following code to your functions.php file

add_filter('wp_dropdown_users_args', 'assign_any_users_author_box_func', 10, 2);
function assign_any_users_author_box_func($query_args, $r){
    $query_arg['who'] = 'any';
    return $query_arg;
}

It should be noted that the user role must also have permissions for the post type that you want them to show up on.