Wordpress Plugins
Private Groups bbPress Plugin
bbPress makes it easy to add a forum to your WordPress community. There are plenty of extensions that let you do even more with it. Private Groups for bbPress is a handy extension that lets you host unlimited private forum groups on your site, which can then be associated to forums.
Users can be part of one or multiple groups. Each forum can be set to public or private. Webmasters have the option to show non-members which forums exist while limiting their access. They can also set up directions for non-group members to help them sign up faster.
This plugin lets you display forum lists, replies and topics for private forums with widgets.