DISCLAIMER: I'm the Relationship Captain in mine. But mine don't go tribal in size -- V's are my speed.
I think that happens in ANY close group -- in our circle of friends I've been the Den Mother. I'm friends with another person who has been the Den Mother too in her spheres of influence. In our joint group it's usually me and her hashing things out and getting the others to MOVE.
I think some people just have those natural skills that have in time grown sharper and better -- so they fall into a a place of "Let's get it organized!" and others cluster around the hub.
If there's 2 silverbacks in the tree that can't share power well, it makes for drama, as Loving Radiance notes. But my friend and I do share power and it's awesome fun -- sometimes she's leading the mission and I love having a break and being just one of the regular bees. Other times I'm the Queen of the Hive and she gets a break. Other times we operate like the Dynamic Duo.
But for new people to be able to join the group -- the style and personality of the de facto "leader" matters. Do they lead from the front and lead the charge? Or do they lead from the back and herd the cats? Some kind of combo thing?