Demetri was born in Greece and transformed by the Egyptian coven's leader Amun. They were very close at the time and worked together to develop his tracking power.
When the Volturi heard of Demetri and his powerful gift, he was offered a place in the guard. He easily obliged due to Chelsea's power over emotional bonds. The Volturi already had a tracker, but once they found out that Demetri's power proved stronger, they had him replaced.