In small bowl combine olive oil, garlic, chili flakes, oregano, rosemary and lemon zest. Whisk together marinade.
Cut chicken into 3/4-inch cubes. Alternate chicken and grapes and thread onto 12 skewers.
Place skewers into a baking dish or pan large enough to hold them. Pour marinade over skewers, coating each one. Marinate for 4 to 24 hours.
Remove skewers from marinade and let excess oil drip off. Season with salt.
Grill until chicken is cooked through, about 3 to 5 minutes on each side.
Arrange on serving platter and drizzle with additional olive oil and lemon juice.