Well this is quite a complicated and sometimes confusing system , on the other hand it does give many many ways to give or deny access
this link could be a good read on this topic:
http://www.phpbb.com/support/docume...permissions.php
Sure black pearl is not missing this feature, I had a look arround in my ACP and I must admit this took me a while too hahaha
One way/example of setting:
ACP>>forum permissions>>tick all forums>>submit>>choose group>>set each forum>>you can check them with view permissions (sorry if after "set each forum" i get unclear but my own pages went down halfway writing this! ..ppfffft unreliable freehosts grrrr )