Get users with atleast one post

global $wpdb;
$min_posts = 5; // Make sure it's int, it's not escaped in the query
$author_ids = $wpdb->get_col("SELECT `post_author` FROM
    (SELECT `post_author`, COUNT(*) AS `count` FROM {$wpdb->posts}
        WHERE `post_status`='publish' GROUP BY `post_author`) AS `stats`
    WHERE `count` >= {$min_posts} ORDER BY `count` DESC;");
// Do what you want to $author_ids from here on...

This will return the User IDs of the Authors with 5+ published posts and orders them descending, by post counts.