<p>Does anyone know of some good resources related to setting up heirarchical user account systems? I'm currently setting one up and am struggling with some of the more complex logic (especially with determining permissions). I was hoping I might be able to find some resources to help me along.</p>

<p><strong>Some Background:</strong>
I'm building a user account system for a web CMS that allows for a nested group hierarchy. Each group can be allowed/denied access to read, write, add, and delete (either explicitly for that group, or implicitly by one of its parents). As if that weren't complicated enough, the system also allows for users to be members of multiple groups. -- This is where I'm stuck. I've got everything set up, but I'm struggling with the actual logic for determining pemissions for a  given user.</p>

Look at the permissions in the
Andrew File System
.  It allows users to create and administer groups of their own, while selectively assigning admin rights and ACLs.  You might find that many of the pesky details are already worked out for you in their model.
Edit:
here's a better link to AFS documentation:
http://www.cs.cmu.edu/~help/afs/index.html
Here's the section on groups:
http://www.cs.cmu.edu/~help/afs/afs_groups.html