how do i export an imovie to a facebook group?

It is easy to export to a Facebook account, but how do I export to a Facebook Group that has restricted access? The Group under my account is a Private Group which means only members of the group can view its content, however from both my iPhone and in iMovie three is no reference to Facebook Groups of any kind.

How can I export my iMovie into my private group without making it public eg through my main FB account or via a service such as YouTube? I just want the file to be in the private group and not visible to non-members.

TIA