I'd like to restrict access to a certain directory in my webroot to all users that belong to a specific group.

I know that I can create a .htpasswd file that specifies a user, or even several users, that should be accepted for HTTP Basic/Digest Authentication, but is it possible to have Apache accept any username/password from the local Linux machine's users? 

To go a step further, is it possible (through .htpasswd or other means that anyone is aware of) to allow any valid user within a specific group?