how to find user ids of all commenters in a post

Question

The get_comments() answer by Poulomi Nag is correct. This will be somewhat more efficient.

global $wpdb, $post;
$query = sprintf("SELECT user_id
                    FROM {$wpdb->comments}
                    JOIN {$wpdb->posts} ON {$wpdb->posts}.ID = {$wpdb->comments}.comment_post_ID
                    WHERE comment_post_ID = %d
                    AND comment_approved = '1'",
                  $post->ID);
$authors = $wpdb->get_col($query);
$authors = array_unique($authors);
$authors = array_diff($authors,array('0')); // Remove those where users are not registered

Another alternative is to use WP_Comment_Query
http://core.trac.wordpress.org/browser/branches/3.2/wp-includes/comment.php#L186