Suppose you want to allow a specific group of users to access particular forums, while forbidding everyone else from those forums.

For example purposes, we'll say the people you want to allow are called 'customers'. Create a 'customers' usergroup at Admin Panel -> Members -> Manage Usergroups (just use the same permissions as regular members unless you want different). Move the desired members to that usergroup by editing their profiles. Then, edit the specific forums you have in mind (either through Admin Panel -> Forums -> Bulk Edit or visiting the forum and clicking edit in the editorial controls area). On the edit forum page, there's a "Display forum permissions for editing" link near the bottom. Click that to expand that area. Then select "no" for "Can view forum?" for the guest and member usergroups, "yes" (or leaving as the default would work too) for the customers group. You now have a forum that can only be viewed by customers.