Creating private groups and categories
You can create private groups for your forum, as well as categories that are only available to those within a particular group.
A category can be hidden to those who are not in a particular group. Access and visibility can be customized for each category, for example some categories could be readable by general users but only those in a particular group could post to it.
Categories that are hidden from a particular user simply do not show up in their view of the forum.