I hope this isn't too off-topic, and I'm not sure if there are any forum rules about discussing a non-ninja extension --but since we've made mention about a 'groups' application from a security perspective, I wonder if any of you have tried using this...
Inline content ACL plugin
http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,5673/Itemid,35/
I like it. If you use CB, you can create a custom field, call it "cb_group" if you like, give it a value like "MyGroup" (whatever), then you can allow/restrict access to sections of your article (including custom loadposition modules for each group, based on the value of the custom CB field. Like this:
(iACL type=cbfield MyGroup,value2}personalized content here{/iACL} - The content between the tags is visible if the value of the user's CB field is available in the list.
or the opposite...
# {iACL type=!cbfield MyGroup,value2}personalized content here{/iACL} - the content between the tags is invisible for the groups specified in the above option
also works for Usernames, ID's etc...
This way you can create any number of user groups just based on CB_group field values and control which group can see what. I use it for personalization -to display a custom site text-logo (using the amazing Ninja SuperSIFR, for each group. I also used it along with a custom field cb_birthday to display a happy birthday message on the proper day for a family website I setup. The real nice thing is being able to load a specific module position based on the cb_group field value!
Hope I didn't bore you with this, in case it's something everyone already knows about.
Cheers.