example.php

<?php
/**
 * Filter the content of the new user registration email
 * @param $message The email content
 * @param $user The user object
 * @param $blogname The name of the site
 * @param $url The url for the user profile
 */
function prefix_new_registration_email_content( $message, $user, $blogname, $url ) {
 $_message = $meessage . '<br>';
 $extra_field = get_user_meta( $user->user_id, 'my_extra_field_name', true );

 $_message = sprintf( '<p>%s</p>', $extra_field );

  return $_message;
}

add_filter( 'wcmo_new_registration_email_content', 'prefix_new_registration_email_content', 10, 4 );

This is helpful. Any other way to add the $user info to 'print'? I can't make it work.