I give mine coffee grounds all of the time with no problems. I have 16 chickens and they typically get about 3 tablespoons of grounds almost every day. I make up a bowl of all of my vegetable waste each day and give this to them. I put in any fruits and vegetable scraps that I have including things that some folks on here say are bad. I think many people overthink what they can give the chickens. In addition to watermelon rinds, broccoli, greens and the like, I also give them coffee grounds, avocado skins, tomatoes, orange peels, potato peels, leftover rice and beans, apple cores, onions etc. If it's a fruit or vegetable scrap, I give it to them and they eat it up and have never had any problems whatsoever. I recently had boiled shrimp and gave them all of the shells. Yep, they loved them.
Why do I say some overthink what they give them? Apple seeds are a good example. Cyanide in apple seeds...yes but it is only trace amounts. Your chickens could not eat enough apple seeds to actually hurt them