Adding grain is kinda tricky if you want it to really have that 35mm printed look - I've worked with students hitting a basic noise filter into their DV or HD film and come crying later that it looks more like a bad TV reception than film. :D
The grain acts as a "pusher" to tones rather than being just noise. I found that layering the noise on top of the footage (in this case with After Effects), blurring the noise with a small radius and using a blend mode like overlay or soft light. Also, if this grain is added before color grading (to give those 35mm tones and removing possible gamma induced errors) it will look more like film.
This method works but it's still not -exactly- like film print grain. And, there's this thing about using After Effects for 4K video. :D Serious renderfarm workshop needed for that, heh.