exsiteie

Just in case anyone is looking for this in the future. This is the solution:

Add to functions.php:

add_action( 'save_post', 'add_authors_name');
 function add_authors_name( $post_id ) {
 global $post;
 $post_author = $post->post_author;  // returns the Author ID
 // get the author's WP_User object so we can get the Author Name
 $post_author_obj = get_userdata( $post_author );
 $post_author_name = $post_author_obj->first_name . ' ' . $post_author_obj->last_name;
 wp_set_post_terms( $post_id, $post_author_name, 'post_tag', true );
 }

Think I might be getting somewhere, but still not quite… anyone?

add_action( 'save_post', 'add_authors_name', 10, 2);
function add_authors_name( $post_id ) {
$post_author = get_the_author($post_id);
$post_author =  str_replace(' ', '-', $post_author);
wp_set_post_terms( $post_id, "$post_author", 'post_tag', true );