on the users_subscriptions.php page you would need to edit the MailingGroups recordset
add a where clause to remove groups you want to be private. for example, the default MailingGroups recordset is:
SELECT *
FROM pm1_mailinggroups
ORDER BY GroupName ASC
you could change that to:
SELECT *
FROM pm1_mailinggroups
WHERE pm1_mailinggroups.GroupName != 'board members'
ORDER BY GroupName ASC
to remove the board members list from the results.