Cool idea! I'm not great at picking specific roles in the group, but I'll give it a go.
Bruce Willis: Strong, smart and Die Hard. Need another reason to make him the leader?
Will Smith: Kicks ass and makes you laugh while he does it. He'd make a good Marco.
Jessica Alba: If Dark Angel is anything to go by, she can be very useful in a fight. Still, I could easily imagine her as the caring one.
Milla Jovovich: Yeerks might not be Zombies, but I'm confident she can defeat them just as well. Definitely a warrior.
David Boreanaz: Couldn't think of anybody else to play the loner and I really liked him in Angel.
Good choices Blue! And the thing they have in common is that they have all played roles in awesome Superhero movies! And snap on the Jessica Alba choice!