UPDATE: 2/2/03
Beta 2 released. New options include: ability to add usergroups who can see all private threads in the admin CP, ability for users to create private threads that only they and selected usergroups can see (ON/OFF option), and a few bugfixes. To upgrade from Beta 1.1, follow the instructions in the upgrade.txt file; to upgrade from Beta 1, follow the instructions to edit editpost.php and postings.php in the install.txt file and then follow the instructions (minus the editpost.php and postings.php parts) in the upgrade.txt file.
Credits for Beta 2: Xenon, as I took a piece of code from his Custom Postbit hack to get this hack to work fully.
PHP Code:
strstr(" ".$canusecpb." ", " ".$bbuserinfo[usergroupid]." ")