grahm-
You'll need to define your requirements a little more thoroughly.

A few example cases:

• Will there be a "group C" that can see content from both "super user A" and "super user B"?  
  • If so, how should the content be presented?
     
• Is the content just text/html or might it include interactive items such as surveys, comments, etc?
   
• Will there be general content for all to see mixed on the same page with the "group only" content?

 
The easiest approach to segregate content within a single portal is to simply create separate pages for your different user roles.  If you set an entire page to be visible only by a certain role, other roles won't even see the page listed in the menu.  Alternately, you can set all pages to be visible by all roles (so they all show up in the menu) but set individual modules on a "role-only" page to be visible only by a certain role.  For other roles, add a text/html module visible to all but the "allowed" role stating something like "you do not have sufficient permissions to see this page's content".  A "deny role" text/html module is good for this -- DNN 5.x already includes it or DNN 4.x users can get a free one here).
  
There are many content segregation approaches you can take with a default DNN installation and even more options if you add certain third-party modules.  You first need to very clearly define how you would like your process to work (consider all possible use cases) and then find a configuration that delivers what you desire.

Post back more specific details on how you would like things to work and you'll probably get some better / more specific suggestions.

-mamlin