Soak 4 (12-inch) wooden skewers in hot water 30 minutes. Meanwhile, place shrimp in large plastic re-sealable food storage bag. Combine soy sauce, oil, garlic, and red pepper (if desired) in cup; mix well.
Pour over shrimp. Seal bag; turn to coat.
Marinate at room temperature 15 minutes.
Drain shrimp, reserving marinade.
Alternately thread shrimp and onions onto skewers.
Place skewers on grid or rack of broiler pan. Brush with reserved marinade.
Grill, covered, over medium-hot coals or broil 5 to 6 inches from heat 5 minutes on each side or until shrimp are pink and opaque.