gistfile1.php

//RAW SQL
SELECT u.ID, u.user_nicename 
FROM wp_users AS u
LEFT JOIN wp_usermeta AS m ON m.user_id=u.ID AND m.meta_key='wp_capabilities'
WHERE m.meta_value LIKE '%contributor%'

//PHP
$sql = "
SELECT u.ID, u.user_nicename 
FROM $wpdb->users AS u
LEFT JOIN $wpdb->usermeta AS m ON m.user_id=u.ID AND m.meta_key='wp_capabilities'
WHERE  m.meta_value LIKE '%contributor%'
";
$authors = $wpdb->get_results($sql);