I used to demo for the developers of the Reduce Grain/Match Grain/Add Grain effects in AE. If you have trouble, I might be able to help you. I’ve found that the default settings (with one exception) do a great job. Most of the other settings are for tweaking.
The exception is: you should enable Temporal Filtering when using footage (not a still). Leaving it unchecked as a default was a holdover from the effect’s history as a Photoshop plugin.