Parboiling your bratwurst will allow them to cook properly on the grill, but there are some details to consider so all that time preparing them doesn't go to waste.
One thing to watch out for is accidentally poking holes into the casing of the sausage, which lets all the juicy flavor escape.
Even though they will stand up to the heat better after being parboiled, high heat can still cause the sausage skin to break if the grill grates are too hot
Go for a medium heat that won't damage the casing but will still allow you to get a nice color.
If plenty of people are coming to your cookout, consider simmering the meat ahead of time.
Because they don't take long on the grill once they are parboiled, you can get a suitable amount ready beforehand and keep them warm until guests start to arrive.