The BHK's friends are, more than likely, not relivant to the actual story-line of the game and will not accompany him on his journey. They're characters like Selphie, Wakka, and Tidus. They're there to give you background and more depth on the character.
I seriously doubt they're altnerates of the people on the Destiny Islands. That theory is bull s**t and I've bitched about it enough for you morons to understand why.