I added 24 oz of cold pressed coffee at bottling time to a True Brew Oaked Imperial Stout kit. After about 3 weeks in the bottles it came out great. I really like coffee though so it might not be for everyone.
I would add coffee to the secondary or at bottling, myself. If you add during primary fermentation, I'd be worried that it could have an adverse affect on the yeast or that you could lose some aroma to the CO2 output.