function wpp_create_dir_for_user( $user_id ) {
$upload_dir = wp_upload_dir();
$user_dir = sprintf( '%s/users/%s', $upload_dir[ 'basedir' ], $user_id );
wp_mkdir_p( $user_dir );
}
add_action( 'user_register', 'wpp_create_dir_for_user' );