What about a slightly more organised system? Doesn't need to be too structured, it can have a leader (kick/invite/promote), an officer (invite) and a member rank for simplicity and functionality.
If you'd like to go with military flavour, they can be called Platoons, the leader can be called a Lieutenant (or a Captain), the officer can be called a Sergeant and the member can be called a Soldier (or Private).
It's a matter of clan tags being unique and all that, so when you'd create one via a simple in-game join/create browser you'd input a unique up-to-5-characters-tag and a name.
