Combine tequila and jalapeño slices in a large pitcher and let sit about 30 minutes (if you’re just not that into spicy margs, you can skip this step).
Remove jalapeños from tequila and add grapefruit juice, sugar and water. Stir to blend and pour into cone-shaped paper cups (you can find them on Amazon) because they like, you know, kind of look like a margarita cup—work with me here. You can stand them up in cups to keep them from tipping over.