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.
Two Kids Brewery
Primary: nothing. :(
Seconary: Don't use one, generally
Kegged: Belgian Brown Ale (loosely based on Corsendonk's Brown Ale); Saison
"For a quart of ale is a dish for a king." - Shakespeare