If I'm flying, I'll gladly pay an extra 30 euros/$40 for a flight that's not insanely early. I'd have trouble getting to sleep the night before, knowing that I have to get up so early, and what if someone in your group can't get going fast enough?
The area around the Grenoble train station isn't the greatest, but I doubt there's many undesirables hanging around at that hour of the morning. But it will be dark, no question about that. As for the safety of your specific route, if you haven't been there already, I'd do a "walk through" via Google Street View and see what it looks like.
1.5km, 15 minutes... any luggage?
Maybe (with the aid of your hotel's personnel) you could arrange for a taxi to drive you directly there--it shouldn't be that expensive, since the distance is pretty short.
The cheapest fares on SNCF don't allow for credit if you miss your train, so you'd be paying full "walk-up" fare to get to your destination... very expensive. Thus you really don't have much room for error; if the cab's late, you're hosed. The only thing you can truly depend on at that hour of the morning are your feet. And even that may be questionable.
