How to use same email for multiple users

You can use wpmu_validate_user_signup filter to remove the error and then define WP_IMPORTING just to skip the email_exist() check in wp_insert_user() function:

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;
}

UPDATE: for a non Multi-site setup try this code:

add_filter('pre_user_email', 'skip_email_exist');
function skip_email_exist($user_email){
    define( 'WP_IMPORTING', 'SKIP_EMAIL_EXIST' );
    return $user_email;
}