1. Add new (blank) layer
2. Add 50% Grey fill, Overlay blending Mode
2. Add Gaussian Noise at level 4 – 8
3. Add Gaussian Blur at .3px
4. Adjust the levels of this layer to accentuate the grain – end points: 50/205
5. Adjust the opacity.

This doesn’t add grain to deep shadows. However, copy the pixel-bearing layer adding the noise & blur to that. Then make a luminance selection and invert it, copying the selection to a new layer (ctrl-j) to add grain to the deep shadows.