Brew tea in 2 cups boiling water 4 minutes; strain or remove bag. Let cool completely. Meanwhile, combine sugar and 1/2 cup water in a small saucepan. Bring to a boil, stirring until sugar is dissolved. Let cool completely, then combine with grapefruit juice and tea in a pitcher. Refrigerate until ready to serve, up to 2 days. Serve over ice, garnished with grapefruit slices.