Use it http://www.onetouchinnovation.com/scripting/extending-bbpress-private-forums , is so easy , but don’t turn on “Hide Private Forums and Topics” cause it have a lot of bugs , don’t use too “add users” or “restrict registration” plugin cause it will reset you private forums , if you need add users from admin panel use integration with worddpress and you will don’t have any problem
sorry for my bad english