So the point is to try and determine who is selfless and who is a self-preservationist? I don't think there's enough info to determine what I would do in that brief instant (and it would likely be very brief).
Do I know these people? Do I like these people? Is there anything else in the room I can wedge into the corner where the grenade landed? Is the grenade's pin actually pulled, thus making sure this isn't a faggy logic test? What is my proximity as compared to the other people's? Are they aware that in a grenade situation, one should become one with the floor?
I ask these questions, because they seriously matter and though you'd think "bullshit, there's no way you'd weigh all those possibilities in that instant," then piss off, because we're not asking about knee-jerk reactions here, in front of computers, about as far from a live grenade as one can be.
Truth is, nobody would truly know what they'd do in that situation. Sure, people who want others to think they're noble, or brave, or selfless would say, "I'd jump on it in a heartbeat." Those who don't give a sh*te or want others to think they're badasses or whatnot would say, "F those other people, I'm out the window."
What I'm saying is, you won't get a serious answer, nor an accurate one. This question is akin to stroking one's ego, or whatever happens to be dangling around at the moment.
That being said, I'll play the game. I'd shout "Down!" whilst belly flopping to the floor myself. Jumping up and out a window would leave you more vulnerable to flying shrapnel if it went off before you got out. Even if you got out in time, who's to say you won't cut something vital on a shard of glass or land on your friggin' head wrong and snap your neck. I say hit the floor because jumping on a grenade will likely just send your bone fragments whizzing lethally around the room anyway.