Ripe passion fruit has a dark purple color and slightly dimpled appearance. Its spicy-sweet flavor goes well with the citrus.
Slice the passion fruit in half over a bowl, catching any juice. Set a fine strainer over the bowl. Using a spoon, scoop flesh into strainer; press down on seeds to extract all juice; you should have 1/4 cup juice. Reserve 2 tablespoons seeds; discard remaining seeds.
Combine passion-fruit juice, orange juice, simple syrup, and 1/2 cup cold water in a large pitcher; stir to combine. Fill pitcher with ice. Pour into individual glasses, and garnish each glass with 1 teaspoon seeds. Serve.