Hi Danny!
Sorry it took me a little bit to get back to you – it took a little bit to track this down! ??
In wp-includes/ms-functions.php, you will find:
$welcome_email = apply_filters( 'update_welcome_user_email', $welcome_email, $user_id, $password, $meta );
$welcome_email = str_replace( 'SITE_NAME', $current_site->site_name, $welcome_email );
$welcome_email = str_replace( 'USERNAME', $user->user_login, $welcome_email );
$welcome_email = str_replace( 'PASSWORD', $password, $welcome_email );
$welcome_email = str_replace( 'LOGINLINK', wp_login_url(), $welcome_email );