I've added a feature to the group system that allows group owners to make the group private. Private groups do not appear on the group list and are not accessible to non-members, even via direct link. In fact, they shouldn't even show up in search.
I've also added some enhancements to the group list page. Groups you're a member of will now float to the top of the list and show that you're a member rather than displaying the Request Membership link. Private groups will also be labeled as such for administrators and plot members, who can invade every group's privacy. :P