A friend of mine was recently in a bar where a woman ordered a Guinness and asked for coffee grounds to be added and went on to explain it was a Boston thing. I tried to find any reference to adding grounds directly into a pint and my conclusion is the woman might have had a few too many. While there are numerous coffee beers and beers which used Roasted Barley and Chocolate Malt to achieve a coffee flavor, AFAIK there is no tradition of putting grounds in the beer. There is absolutely nothing worse when drinking your daily go-go juice than taking the last sip and getting a mouthful of grounds. You immediately begin to wretch uncontrollably. I can imagine in a beer the sensation may be even less fulfilling. So no, do not put coffee grounds in Guinness even if some random woman from Boston has it as a tradition it is an incredibly bad idea.