In a private discussion group should always be private, in that one can only @mention members of the group. If one @mentions a non-member, Clarizen should generate an error message and prevent the post, which is does.
However, you can bypass this restriction by simply creating a group (say Bogus-Group) of one person and @Bogus-Group and Clarizen does not prevent that post from continuing. This is inconsistent behavior at best and a way to bypass the "Private" nature of the group, i.e. a security hole.
e.g. a bad scenario of this security hole would be an executive sharing financial info with the company or individual outside the private group because the "Privacy" business rule was not enforced.
A warning is not even provided that one is about to post a message outside of the group membership.
This is a work-as-designed feature that seems inconsistent at best, and a security hole at worst.
This was temporarily considered a bug but is not considered a feature request: https://success.clarizen.com/hc/en-us/requests/28773