Multiple communities under one site

I am trying to host multiple commnities under one site. What I mean by this is that anonymous users will see general news and other topics, but a registered user will see only items specific to his or her group. My questions are:

1. How do I limit what different groups see?
2. How do I specify group access during login registration?

I have tried user roles and taxonomy access control. This has brought me close but not quite there. Does anyone know of a step-by-step procedure to make this happen? Do I need multiple drupal installations to make this happen? I would prefer for a user to be able to register with one ID and be able to access items from one or more groups if he or she is a member of those groups.

Thanks in advance for your help.