add_filter( 'groups_import_export_new_user_registration_subject', 'example_groups_import_export_new_user_registration_subject', 10, 3 );
function example_groups_import_export_new_user_registration_subject( $subject, $user_id, $plaintext_pass ) {
	$subject = 'Here you can add whatever you like to display as subject, in the notification mail a user receives for his new account';

	return $subject;
}

add_filter( 'groups_import_export_new_user_registration_message', 'example_groups_import_export_new_user_registration_message', 10, 3 );
function example_groups_import_export_new_user_registration_message( $message, $user_id, $plaintext_pass ) {
	$message = 'Here you can add whatever you like to display as message, in the notification mail a user receives for his new account';

	return $message;
}