Preheat oven to 425°F. Wrap individual beets in foil and seal tightly. Place on a baking sheet and bake until tender, about 1 hour.
Meanwhile, prepare fried rosemary by heating 2 tablespoons of oil in a small skillet over medium heat. Add rosemary and cook, stirring often, until sizzling subsides, about 15 seconds. Using a slotted spoon, transfer to paper towels and let drain; season with salt.
About 20 minutes before beets finish roasting, prepare the couscous by bringing broth to a boil in a small saucepan. Add couscous and stir. Cover with a tight-fitting lid and reduce heat to simmer for 15 minutes, until couscous is tender and liquid is absorbed. Remove from heat and fluff with a fork. Place lid on and set aside.
Remove beets from oven and let cool slightly before peeling skin off. Chop into 1-inch cubed pieces.
Add beets and couscous (which should still be warm) to a large serving bowl and toss well. The color of the couscous will change to a vibrant pink as you continue mixing.
Add grapefruit juice, shallots, and vinegar; toss to coat. Season with salt, to taste. Let sit for 15 minutes to allow shallots to soften slightly. To serve, drizzle with more oil, toss, and top with fried rosemary.