Lightly brush the steaks with oil and season evenly with 2 teaspoons of the salt and the pepper. Allow the steaks to stand at room temperature for 15 to 30 minutes before grilling.
Prepare the grill for direct cooking over medium heat (350° to 450°F).
In a small saucepan over medium heat, heat the butter and garlic until the butter melts and comes to a boil. Remove from the heat and let stand for 5 minutes.
Place the shrimp in a medium bowl and toss with 2 tablespoons of the garlic butter; reserve the remaining butter. Season the shrimp with the lemon zest, the remaining ½ teaspoon salt, and the red pepper flakes.
Brush the cooking grates clean. Grill the steaks over direct medium heat, with the lid closed as much as possible, until cooked to your desired doneness, 11 to 13 minutes for medium rare, turning once (if flare-ups occur, move the steaks temporarily over indirect heat). Remove from the grill and let rest for 3 to 5 minutes.
Grill the shrimp over direct medium heat, with the lid closed as much as possible, until they are firm to the touch, lightly charred, and just turning opaque in the center, about 5 minutes, turning once.
Reheat the butter. Serve the steaks warm topped with shrimp, butter, and parsley.