Automatically assign a co-author based on current user

gistfile1.aw

<?php
/**
 * Automatically assign a co-author based on current user's usermeta value
 * This code is untested and for demonstration purposes
 *
 * @see http://wordpress.org/support/topic/plugin-co-authors-plus-dynamically-assign-co-authors?replies=1
 */
add_action( 'save_post', 'capx_auto_assign_author' );
function capx_auto_assign_author( $post_id ) {
	global $coauthors_plus;

	if ( !is_object( $coauthors_plus ) )
		return;

	// Get the current user, see if they have a parent_user,
	// and automatically assign the parent user if it exists
	$current_user = wp_get_current_user();
	if ( $parent_user_login = get_user_meta( $current_user->ID, 'parent_user', true ) )
		$coauthors_plus->add_coauthors( $post_id, array( $parent_user_login ), true );

}