Restrict Group Visibility
You can make groups private by using the Restrict Newsgroup Visibility option when you add or edit properties on the Directories tab in ISM. Because private groups are not visible to users who do not have permission to read or post articles, there is no access to those newsgroups or the information they contain.

Note  For best performance, it is recommended that you disable the Restrict Newsgroup Visibility option on virtual directories where anonymous access is allowed. Otherwise, the server will unnecessarily check permissions for each newsgroup when processing the LIST command.