Writing a loop and going through every comment is the option that I can offer you. Take a look at this example:
function get_comment_authors($post_ID){
// Get every comment on this post
$args = array(
'post_id' => $post_ID
);
$comment_objects = get_comments( $args );
// Define an empty array to store the comments
$author_array = array();
// Run a loop through every comment object
foreach ( $comment_objects as $comment ) {
// Get the comment's author
$author_array[] = $comment->comment_author;
}
// Return the authors as an array
return $author_array;
}
So, get_comment_authors(123);
will return every author that left a comment on the post that has an ID of 123.