Can you parfry hash browns in advance? I like making and especially eating hash browns, but they take quite some time to cook. Could I make all of my hash browns in advance by cooking them but not all the way and then finishing them off right before I want to eat them?

I'd cook them first till they are cooked through and then finish them off till they are crispy. Are there any difficulties with this approach?

Is this the way you could make them for a large group of people?

(Just to be clear, I mean the grated kind.)
Par-cooking potatoes is an extremely common technique.

However, the shredded or grated kind of hash browns would be very difficult to par-cook with wet methods, as they would go to mush quite quickly if you tried to simmer them, and then you would have the trouble of squeezing out the water afterwards (which is necessary so they crisp up, and so they finish cooking rapidly).

Par-frying would be labor intensive, and not terribly scalable either.

Typically, shredded hash browns only take about 15 minutes to cook, which is not terribly long.

I have had some success with making shredded hash browns in a very hot (450 F, 230 C) oven, baking them for about 30 minutes.  They are not quite as good as fried, but they are still good, and the technique would scale quite well.

If you are willing to experiment, I believe that par-baking them for about 20-30 minutes at a more moderate temperature (say about 350 F, 180 C) would be a promising approach.  They would stay dry, and you could then finish-fry them just to crisp up and get some buttery flavor.  You would need try it a few times to work out the be combination of baking and frying times.