I have been of the belief that Charlotte, Daniel, Miles were chosen for their specific abilities and or connections with the island.
Miles- there are dead bodies on the island it is know and who better than a ghost whisperer to pick the secrets up and while you we are at it if the whispers are spirits who better to pick them up. We learned that when he walked over the bodies of Danielle and Karl.
Charlotte- well we know she knows something about the Dharma project as she went to the dig where she saw the Dharma symbol. I think she may be to young to have been part of Dharma, but if she doesn't know where she was born, then she could be a child from someone who was on the island. Her background information is limited, but my other thought she is there for a variety of reasons.
Daniel - he was there before and I just watched the clip from this show (and not knowing where to put this, as i didn't see anything in here for this) but Daniel says i am not sure if I can do this. She encourages Daniel he can remember and do it. Remember how would he forget and how would he specifically know about what they are trying to do!
I am of a strong opinion that Daniel Faraday was Dharma and somehow was able to leave. Somehow Daniel has travelled to much and it has effected his mind. IMHO
