Instead of adding coffee beans, why don't you brew some coffee and add that?
If it is mostly aroma you are looking for, I don't know that you need to worry much, if fermentation is done. You might consider putting your beer in secondary, chilling it, then adding the beans. This way, you've shut the yeast and any contaminants that might be on the beans.
In any case, I would suggest waiting until fermentation is complete. Then you won't need to worry about contamination.
Brody's Brew House
Primary: Rye Saison
Seconary: Imperial Stout
Kegged: Pale Ale
In the fridge: Genesee IPA