Eating a few bad grapes will give you a stomach ache or diarrhea but eating a few bad grapes should not harm you other than the stomach ache or diarrhea.
Grapes that are bad will have a sour smell to them and have a bad taste to them as well as may have mold on them and you would likely not want to eat anymore bad grapes after ingesting one or two.
So you shouldn't get sick other than the upset stomach or diarrhea from the bad grapes.
I've accidentally eaten a bad grape and it only gave me a stomach ache and stomach cramps but no diarrhea since I didn't eat too many bad grapes.