From fairly early on, Pinax had tribes and projects. The intention of the distinction was that tribes are a loose grouping of people with a common interest (see Seth Godin's great little book Tribes: We Need You to Lead Us) whereas projects were more focused around managing a group of people working on common tasks.
This distinction was reflected in the differences in implementation: projects are invite only, tribes are open to anyone. Projects have tasks, tribes don't. But there are a lot of similarities: both have threaded discussion, wikis and photo pools. There's also a lot of duplicated code.
It wasn't long before I realised that really projects and tribes we just two subclasses of the same class (or instances of the same metaclass—more on that in a moment). So one of the things we're working on for the next release of Pinax is to merge the two into a single model but then allow a site developer to create as many differently configured subclasses/instances of this model as they like.
We haven't worked out the details yet, but basically it would allow you to define a new subclass/instance of "group", pick the membership model and what apps and object types can be attached to it.
Quite separately, I've noticed that there is another kind of structure that tacitly exists between tribes on Cloud27. I started a Python tribe but others have started Ruby and PHP tribes. There are also geographic tribes and language-based tribes. So there is another sense in which a set of tribes could be labelled as "programming language" tribes, or "geographical" tribes or "natural language" tribes.
So we have:
To slightly complicate things, one could imagine an "Australian Pythonistas" tribe. Or various intersections of "movies", "food", etc with geography-based tribes (e.g. Italian Food, French Films). These sorts of semantic relationships seems quite orthogonal to defining a type of group that has a wiki but not threaded discussions or is invitation-only versus free-for-all.
I'm trying to wrap my head around which to view as instances versus subclasses. As a longtime data modeler, I thought I had a grasp on this stuff but my brain is hurting :-)
Any suggestions for a metamodel?