A lot of groups coordinate on discord; they'll have a template & an area they want to target. Some of the larger groups may have thousands of people. It depends on the group, but in one I've seen, they organize people by birthdate for some of the designs, and by color for the larger part of the design, and let people go.

There's a way you can get an overlay of the intended design on the r/place canvas, supposedly, but it didn't work for me. But like they'll have templates or whatever.

One of the largest groups is "the void" and normally it just tries to black out the canvas. This year it looks like they're helping out the fuckspez black and white design mostly.