You get more XP from doing team orientated things than you do for kills. A medic can rack up loads more points healing allies than he will killing enemies, the spec ops also receives loads of XP from giving ammo, i think the soldier gets points for absorbing damage into his shield that would otherwise hit an ally but i'm not sure on that one yet.
You do also lose points for dying but it isn't much at all, around 100 i think. Also i don't think you lost any points if someone revives you.
[EDIT] RAWR! Just completed the coop level with only 3 people in our team, it was on a knifes edge most of the time but we just about pulled it off. The tactic we used was me constantly ammoing the soldier to keep his shields up while spamming the secondary fire with the marksman all while me and the medic would hide behind the soldiers shield, the key was for the soldier to do his best at keeping his movements predicable so me and the medic could stay right behind the safety of his shield. When my invisibility was charged the other two would hang back somewhere while i ran out and took as many down as i could before my adrenaline ran out, which usually yielded about 5-6 easy kills. Really difficult, especially when the chimera rushed you, but we got an insane amount of XP out of it. The solider in particular got a mental amount of XP due to the amount of damage his shield must have absorbed.