I think maybe the others have been posing as dharma for along time so they can gather outside info, get food drops, and keep other dharma people from coming. I believe naomi was from dharma sent there to find out what had happened, there would be no other way to communicate because of the jamming equipment. maybe that is why locke killed naomi because he now knew the island needed protection from the outside world. (jacob/walt?) told him he had work to do.