To add the grain effect, go to the Filter menu and select Noise > Add Noise. Click on the Gaussian button, since it gives a more realistic distribution, make sure the preview box is checked and then move the slider until the amount of noise looks appropriate. As a rule of thumb, I've found that a percentage value roughly 1/100th of the width of the image in pixels seems to work well. My image here is about 3,000 pixels wide, so I'm using a setting of 30 percent.
The noise that the filter adds looks far too hard and digital to be mistaken for film grain, so we need to soften it slightly. Go back to the filter menu and select Blur > Gaussian Blur. You only need a slight touch of this effect, so set the slider to around one pixel. This softens the blur effect slightly, making it look less mechanical.
Already the effect looks very much like film grain, but it can be improved by a couple of additional tweaks. First, use the Image > Adjustments > Brightness/Contrast sliders to increase the brightness and slightly reduce the contrast. The actual amount depends on the existing tone of the image, so you'll have to do this part by eye.
Finally you may find that your results look better if you reduce the opacity of the grain layer slightly, say to around 80 percent. This makes the effect more subtle like a slightly faded print.
And there we have it, photographic technology taken back 40 years in just ten minutes, and instant atmosphere added.