How to use same email for multiple users? [duplicate]

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.

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