You need a crew to help you out, but you have to do things to get their attention.

Effect - Code

Colin - Defeat Wolf
Colin (Drafter) - Defeat the "TFK" gang.
Neville - Finish the Cross/Police chase at the start of the game.
Nikki - Defeat Angie, Wolf and Kenji.
Nikki (Drafter) - Beat all three "gangs"
Sal - Complete 4-6 races
Sal (Scout) - Complete 4-6 races
Sampson - Defeat Angie
Sampson (Blocker) - Defeat the "21st Street" gang.
Yumi - Defeat Kenji
Yumi (Scout) - Beat Bushido