Content Management and User/Group Management

I'm curious if there are some recommendations for an open-source
content management that also has robust user/group management features.

I've been looking at several systems out there - Mambo has the basic
simplicity I need from the CMS standpoint (my content is not very
dynamic, but there is a tiered and distributed authoring/editing
component) but seems to woefully lack in the area of user/group
management.

I have the need to closely manage subscribers based on their membership
in specific subscription groups. Basically, an organization subscribes
to our service and gets licenses for specific facilities within that
organization. I need to manage the organization as a group, the
facilities (sub-groups) and the users within each facility.

Any pointers toward some candidate solutions would be much appreciated.