Ok, lets see...
The Tribe was something of an accident actually. During the 'bonfire massacre' in the 87 Lost Boys David is interrupted while feeding and does not fully kill his victim and does not dispose of the body in the bonfire.
Mostly dead, Shane crawls to the sea and kills a shark, turning within minutes of being bitten. He overheard David tell Michael what they were and was only too eager to join, yet The Lost Boys left before was fully recovered.
So is any of that Canon?Edit
Of course all of this comes from the highly questionable comic books so while the 'best' explanation we have, it's canon status in debated among fans.
note- comic page here
A very excellent alternative story to Shane's origins was written by a very good friend of mine that I personally prefer.