Unlocking crew members:

You need a crew to help you out, but you have to do things to get their attention.
Sal - Complete 4-6 races
Yumi - Defeat Kenji

Hint: More Money
If you have a previous NFS: Most Wanted save on your memory card you get a bonus $10,000 when you start a career in career mode.
Code

Type the code at title screen:
Unlimited Nitrous Left, Up, Left, Down, Left, Down, Right, Square
Unlimited Speedbreaker Down, Right, Right, Left, Right, Up, Down, Square
Unlimited Crew Charge Down, Up, Up, Right, Left, Left, Right, Square
Unlock NFS:C Vinyl Right, Up, Down, Up, Down, Left, Right, Square
Unlocks Cardboard Box and Driver Vinyl Up, Up, Down, Down, Down, Down, Up, Square
Extra money Down, Up, Left, Down, Right, Up, Square, Triangle