<?php
// Get the posts
$posts_by_author_ID = get_posts( array(
     'post_type'   => 'model'
    ,'post_status' => 'publish'
    ,'orderby'     => 'post_author'
    ,'order'       => 'DESC'
) );
# >>> DEBUGGING STEP 1
echo '<h3>THE OUTPUT OF THE POSTS QUERY</h3><pre>';
print_r( $posts_by_author_ID );
echo '</pre>';
# / STEP 1

foreach ( $posts_by_author_ID as $post )
{
    static $user;

    $new_user = get_user_by( 'id', $post->post_author );

# >>> DEBUGGING STEP 2
echo '<h3>THIS IS ONE OF YOUR USERS</h3><pre>';
print_r( $new_user );
echo '</pre>';
# / STEP 2

    // Let's save some queries
    if (
        isset ( $user )
        AND $user->ID === $new_user->ID
    )
        continue;

    $user = $new_user;

    foreach ( $user->roles as $role )
    {
        $posts_by_author_role[ $role ] = array(
             'premium-models' => $user
            ,'post' => $post
        );
    }
}
// Sort by key a.k.a. role
ksort( $posts_by_author_ID );

# >>> DEBUGGING STEP 3
echo '<h3>THIS IS THE RESULT OF SORTING THE POSTS BY AUTHOR</h3><pre>';
print_r( $posts_by_author_ID );
echo '</pre>';
# / STEP 3

# >>> DEBUGGING STEP 4
echo '<pre>'; // Only for debugging/developing - should get deleted when done
foreach ( $posts_by_author_ID as $role => $data )
{
    list( $user, $post ) = $data;
    // Now we can output our data
    // The user data is saved inside $user
    // The actual post data is saved inside $data

    // The following is only for debugging/developing - should get deleted when done
    var_export( $user, false );
    var_export( $post, false );
}
echo '</pre>'; // Only for debugging/developing - should get deleted when done
# / STEP 4