Preheat a gas grill to high for 20 minutes. Place butter in a large serving bowl.
Place sweet potatoes on a baking sheet and toss with oil, salt and pepper. Lower heat to medium, place potatoes around edges of grill and cook, turning once, until soft when pierced with a skewer, 8 to 10 minutes total.
Transfer potatoes to bowl, sprinkle with parsley and gently toss to coat. Season with more salt if desired; serve warm.