1Mix all Teriyaki Marinade ingredients. Fold thin end of each pork tenderloin under so pork is an even thickness; secure with toothpicks. Place pork in resealable plastic food-storage bag or shallow glass or plastic dish. Pour marinade over pork; turn pork to coat with marinade. Seal bag or cover dish and refrigerate at least 8 hours but no longer than 24 hours, turning pork occasionally.
2Brush grill rack with vegetable oil. If using charcoal grill, place drip pan directly under grilling area, and arrange coals around edge of firebox. Heat coals or gas grill for indirect heat.
3Remove pork from marinade; reserve marinade. Cover and grill pork over drip pan or over unheated side of gas grill and 4 to 5 inches from medium heat 25 to 30 minutes, brushing occasionally with marinade and turning once, until pork is slightly pink in center. Cut pineapple slices in half. Add pineapple to grill for last 2 to 4 minutes of grilling, brushing with marinade and turning once. Discard any remaining marinade.
4Remove toothpicks from pork. Cut pork across grain into thin slices. Serve with pineapple.