you can never join them, enclave kill everyone whos not them, they even eradicate vault even thou they are mutation free... usualy
I figure they'd be desperate enough at this point to accept some outside help. With all their advanced technology, you figure they'd have some way of purifying someone without killing them. If not, maybe they could use you like a mercenary without actually having direct contact with you until you earn their trust by completing high profile missions. I'm still hoping, since no one has ever actually been able to work with the Enclave before.