this is big for the split screeners, who have their friends over or play with other members of the family.
when playing with another person, turn a second controller on and press x to add another controller into play, then sign in your second controller as another account off your ps3.
you can only have a splitscreen of two when doing this method though, you can sign in more but they dont appear
I've just found this video - LINK
Looks like you could play split screen with up to 4 players, I don't know if this was in previous CoDs as I never tried it!