Preheat a grill to medium-high. Fill a very large (5-gallon) stockpot halfway with water and place on the grill. add the beer, onions, lemons, garlic and 3 tbsp. old bay; bring to a boil.
Add the kielbasa and potatoes, reduce the heat and simmer until the potatoes are tender, 15 to 20 minutes. Using a large sieve, transfer the kielbasa and potatoes to a large platter. Cover with foil to keep warm.
Increase the heat to return the liquid to a boil. Using tongs, carefully add the crabs to the pot. put the corn on top of the crabs, then cover and cook until the crabs are bright red and the meat is flaky (remove a claw and crack it to check the meat), about 12 minutes.
Place the corn on the platter with the kielbasa and potatoes. Discard the onions, garlic and lemons. Using a large sieve, transfer the crabs to a second large platter or pile them right on the table. Sprinkle everything with old bay and sea salt.