As far as Larry is concerned, "version 1" of groups/clans will essentially have two roles: leader, and user. We'll likely expand it later based on feedback, but that's what we're going to start with.
In his words (paraphrased)
basically a user will make a group, name it, and other users can join. The leader can give someone else 'leadership' of the group if they choose, but it means they won't be a leader any more.
We'll have to make sure, or course, that the user you're giving leadership control to has the necessary requirements to be a leader (it'll be the same as what's needed to create a group in the first place).