A most excellent essay on groups by the amazingly insightful Clay Shirky.

Here are two lists stolen from there.

Things you must accept about online groups:

• The social and technical are deeply entangled, you can’t partition them.
• Members are different from users. “… the group within the group that matters most.”
• Sometimes the rights of the core group trump those of individuals.

Things you really really ought to design for:

• Participants must have handles that can enable continuity, aggregate reputation, and be discarded
• You will need some scheme that recognizes members in good standing: handle, member since, introduction, karma points.
• You need barriers to participation, so the core can defend the group.
• Some protection against scale (or the tragedy of the commons)

I’d add that there needs to be a shared enterprise, often a document, around which the group revolves. But that’s probably optional.