My dogs eat grapes on the regular.
2 labs and a Boston terrier. Also my chickens devour them with glee.
Grapes can be toxic to dogs, IMO it is not worth the risk. I haven't looked it up recently so I'm not sure if they figured out why yet. Some dogs are not affected, some dogs can go into renal failure after eating one grape. Some dogs can eat a dozen before they get sick. But why risk it? Is their health/life worth a few grapes? There are plenty of safe treats you can feed.