One or two won't hurt as long as they don't appear on the menu regularly but it's better that dogs don't get a taste for them so, if they get a chance, they don't wolf a whole bunch. Tooo many grapes or raisins can cause vomiting or diarrhoea, which can progress to kidney failure in some dogs. This kidney failure is often fatal.
Other common human foods which can make dogs ill include onions, chocolate and avocados. If your dog has eaten any of these, you should consult your veterinarian. Let them know how much your pet has eaten, how much your dog weighs and whether he or she is showing any signs of being unwell.