As you assumed - the solution is pretty easy.
First create a file(for instance) skip-email-check.php
and inside it put the code from the other answer:
add_filter('wpmu_validate_user_signup', 'skip_email_exist');
function skip_email_exist($result){
if(isset($result['errors']->errors['user_email']) && ($key = array_search(__('Sorry, that email address is already used!'), $result['errors']->errors['user_email'])) !== false) {
unset($result['errors']->errors['user_email'][$key]);
if (empty($result['errors']->errors['user_email'])) unset($result['errors']->errors['user_email']);
}
define( 'WP_IMPORTING', 'SKIP_EMAIL_EXIST' );
return $result;
}
Then save this file on your computer and upload it to the /wp-content/mu-plugins/
directory of your WordPress site. If the directory mu-plugins
doesn't exist in the wp-content
directory, simply create it and upload the file there.
This directory is for "must-use plugins" - meaning that any plugins(stand-along files, or plugin directories) found there will be automatically activated for all sites on a network and can not be deactivated, until they are removed from that directory. You can use this directory on a normal(single-install) WordPress and again any plugins you put there will be always activated.