Here’s how you regrain in AE manually:
First you need to set your project to 32 bpc. This technique will only work in float because we need to use some negative color values here. Then degrain your plate. Neat Video is by far the best tool there is for this. Among other reasons, it keeps everything nice and sharp. It is one of the few 3rd party plugins I swear by and totally worth the money. Not sure if Red Giant’s Denoiser works in 32bit, but Ive never found it to be that good anyway, and AE’s built in Remove Grain definitely isn’t 32.
Next you want to duplicate your comp. Name that new comp “Grain”. In there duplicate your footage layer and delete the degrain effect from the bottom layer. Then select Subtract for the color mode for the top layer. Your image should go black now. If you want to see that it’s working, crank up the exposure in the view window and you should see all the nice grain from your image.
Now drop this “Grain” comp into your main comp on top of your degrained footage, and set it’s color mode to Add.
And that’s it! Regrained footage that matches your source exactly.