/**
 * Add a user to network sites.
 *
 * @param int $user_id User ID.
 */
function wpdocs_add_user_to_network_sites( $user_id ) {

	// Put common sites here
	$blogs = array( 1, 2, 3, 5,8 ); 
  
	foreach ( $blogs as $blog ) {
		add_user_to_blog( $user_id, $blog, 'subscriber' );
	}
}
add_action( 'user_register', 'wpdocs_add_user_to_network_sites' );