Preheat the grill to medium high heat, about 400 degrees Fahrenheit.
Fill a large pot with water and bring to a boil.
Clean and cut potatoes in quarters. Add the potatoes to the boiling water, cook for 8 minutes.
While the potatoes are boiling, Clean and cut corn. After 8 minutes add the corn to the boiling water with the potatoes. Cook for another 5-6 minutes or until potatoes are just barley fork tender.
Dump the potatoes and corn in a strainer to drain water, then put in a large bowl.
Add the shrimp, sausage, and onions to the bowl with the potatoes and corn and mix lightly until just combined.
In a microwave safe bowl melt the butter. Add the garlic, seafood seasoning, salt, pepper, and parsley to the butter and mix until combined.
Pour the butter mixture into the large bowl with the potato mixture and mix until combined.
Tear off 6 pieces of foil about 16 inches long and spread them out on a flat surface.
Divide the boil mixture evenly into the 6 pieces of foil. Fold the foil over the mixture and make sure all the edges are sealed.
Place the foil packs onto the grill. Grill for 12-14 minutes, flipping halfway through, or until shrimp is cooked through.
Serve immediately.