Whisk the beer and garlic together in a bowl, toss in the shrimp, then pour everything into a resealable plastic bag. Squeeze out excess air, and seal the bag.
Place the bag into a bowl to catch any leaks, then marinate in the refrigerator for at least 2 hours.
Heat a cast iron grill pan over medium-high heat until it begins to smoke.
Remove the shrimp from the marinade, and shake off excess. Discard the remaining marinade. Cook the shrimp until the shells turn pink and they are no longer translucent in the center, about 5 minutes.
