In a small saucepan, set over low heat, mix the water and sugar together until the sugar dissolves. Set aside to cool.
Next, juice the grapefruits and run the juice through a strainer to catch any pulp. If you're one of those weirdos who likes pulp, then by all means skip this step! Transfer the grapefruit juice to a pitcher and pour in the simple syrup. You should have about 2 cups. Transfer to the fridge to chill for 1 hour. You want this drank cold!
After the grapefruit mixture has chilled, divide between four glasses (about 1/2 cup per glass). Top with cold beer and garnish with a slice of grapefruit.