Select the 'Community & Extras' option at the 'Main' menu and highlight the 'VIP' selection. Then press the Y button and enter one of the following codes (without the quotes) to unlock the corresponding special forces unit.
Unlock Russian Spetsnaz Guard Brigade Enter the code 'SPZA39'.
Unlock Special Spetsnaz Battalion Enter the code 'SPZT17'.
Unlock European Enforcer Corps Enter the code 'EUCA20'.
Unlock U.S. Joint Strike Force Enter the code 'JSFA35'.