A group of crows is called a "murder" of crows, I propose a group of grackles simply be called a "grackle". This could cause confusion, when I say I saw a grackle today, you might wonder whether I was refering to a single grackle or a single group of grackles. I am comfortable with this.
I saw a grackle today, there must have been 5000 of them. They started off down the hill from my house in Hall County, GA. They moved like a confused wave up the hill to the woods at the edge of my property, then back down. They were within earshot all day, and finally took over my lot for about 20 minutes sometime in the afternoon. It's fall, so there are many leaves which are ready to drop with only the slightest provocation. As the grackles hopped about through the canopy, they propogated shorts burst of leaf drop. The squirrels stopped what they were doing and waited for the grackle to move on.