how can i throw back granades?
"Special Traits
The grenade is thrown not shot. This means that the strength of the throw (distance) can be controlled, by how long the Fire button is held down before releasing.
Grenades will detonate on impact with an enemy
Grenades will bounce and roll when thrown.
Grenades can be picked up and thrown back at the player
Secondary weapon use will throw this in an under-hand toss for going under things "

i think this only means that enemies can throw them back at you in the single player. i don;t think that you can actually pick them up and throw them