Uhmmm.... Well I don't know how they would have contacted your character as each individual would be different. Though the rather standard issue for contacts would be...

-An agent (or two given how strong the person is) would arrive in a location your character seems to frequent.
-They would have a brief case with them filled with documents about you, showing their knowledge and resourcefulness.
-They would then begin to explain how they are trying to make a group of like minded people with special strengths to defend against the evils of the world.
-They would then offer something special to your character that through research has lead them to understand what your character desires. Whether its money, status, freedom from past crimes, etc.