You need to edit BoardIndex.template.php (in the default theme directory) and find the below line and replace it with the new one.
', comma_format($context['num_guests']), ' ', $context['num_guests'] == 1 ? $txt['guest'] : $txt['guests'], ', ', comma_format($context['num_users_online']), ' ', $context['num_users_online'] == 1 ? $txt['user'] : $txt['users'];
', comma_format($context['num_users_online']), ' ', $context['num_users_online'] == 1 ? $txt['user'] : $txt['users'], ', ', comma_format($context['num_guests']), ' ', $context['num_guests'] == 1 ? $txt['guest'] : $txt['guests'];
I actually think we should do this by default, not starting the list with members seems to prioritize guests over them.