Marinate Chicken: Put 1/4 - 1/3 cup soy sauce and 6 boneless skinless chicken thighs in a large bowl or storage bag to marinate for 15-30 minutes.
Prep: Heat grill to a medium-high heat and make sure the grill grates are clean.
Cook Chicken: Remove the chicken from the marinade, allowing excess liquid to drip off, and season with salt and pepper. Place chicken thighs directly on the hot grill grates and cook for 2-4 minutes per side or until they reach an internal temperature of 162 degrees F (use a meat thermometer). With about 2 minutes of cook time remaining, brush chicken with teriyaki sauce.
Let Rest: Carefully remove the chicken from the grill and let rest for about 5-10 minutes.
Chicken Breast: feel free to use chicken breasts instead of thighs. They would require a longer cook time of 4-5 minutes per side over medium heat. Then move them to indirect heat to finish cooking for 5-7 minutes or until they reach an internal temperature of 162 degrees F at the thickest part of the meat (use a meat thermometer).
Storage: keep leftovers in an airtight container in the fridge for 4-5 days.