Taken from answerhub:
You can add film grain by adding a post-processing volume to your scene. If you look at the top left of the screen, you should see the tab “Visual Effects”. Click it, and drag the tab to the right of it that says “Post Process Volume” onto your scene. Once you’ve done that, at the bottom left of your screen, click the dropdown that says “Post Process Settings”, select “Scene Color”, and then check the box next to Grain Intensity. There you can input a value from 0-1 of how opaque you want the grain to be; I tend to use 0.5.
Make sure when you add the post process volume that you have the box at the bottom of the details labeled “enabled” checked, for obvious reasons, and “unbounded” checked. The “unbounded” option allows you to put the volume anywhere and have its effects run throughout the entire level, which is true of P.T. so you should be fine there.
You can also find other post processing effects under the volume as well; a few ones that I know were used in P.T. are chromatic aberration (which is labeled as Fringe Intensity), bloom, and color grading (which is a little more tricky and complicated; you can find out more about that here in the UE4 documentation).
Best of luck.