Way to count the number of people who have commented on a post?

by

You could try to count the number of unique comment author emails per post:

/**
 * Number of unique comment author emails per post:
 *
 * @see    http://wordpress.stackexchange.com/a/168606/26350
 * @param  int $pid
 * @return int
 */
function get_unique_commenters_by_post_id( $post_id )
{
   global $wpdb;
   $sql = "SELECT COUNT(1) as uc FROM ( 
              SELECT COUNT(1) as c FROM {$wpdb->comments} 
                  WHERE comment_post_ID = %d 
                  GROUP BY comment_author_email
           ) as t";
    return $wpdb->get_var( $wpdb->prepare( $sql, $post_id ) );
}

Usage example:

echo get_unique_commenters_by_post_id( $post_id = 213 );