On our server we have a few kingdoms in play. Every player starts on adventure mode and has to ask an op to be allowed to play on survival mode (currently this is one of our security measures). A player is always welcome to help with a job, creation or roleplay, as long as the player who is being helped agrees.
Eventually it is even possible that we allow to give a player his own kingdom with its own banner colors and he will be able to claim lands for himself. Of course it's free to switch sides, but we'd like to create an atmosphere of helping and joining with each other more visual and contextual (e.g.: making it like you're in control of your little kingdom).
This is an overview of the current kingdoms on our server: