Get names of authors who have edited a post

The WordPress function get_the_modified_author() will give you the
author who last edited the current post.

But if you want to list all the users that have edit the current post, you could try:

function get_the_modified_authors_wpse_99226(){
    global $wpdb;
    $authors = array();
    $results = $wpdb->get_results( $wpdb->prepare("SELECT post_author FROM $wpdb->posts WHERE (post_type="%s" AND ID = %d) OR (post_type="revision" AND post_parent = %d) GROUP BY post_author", get_post_type( get_the_ID() ), get_the_ID(), get_the_ID() ) );
    foreach($results as $row){
          $authors[] =  get_the_author_meta('display_name', $row->post_author );
    }
    return implode(", ", $authors);
}

This function will give you a distinct comma seperated list of the users that have edit the current post (i.e. their display name). This should also work with custom post types.