Preheat a grill on medium-high heat. Slice 2 of the limes and 1 of the lemons into rounds. Set aside.
Cut the oranges and the remaining 6 limes and 4 lemons in half crosswise. Pour 3/4 cup of the sugar in a bowl. Dip the cut side of each fruit in the sugar to lightly coat.
Grill the sugared fruit over medium-high heat until caramelized with char marks. When cool enough to handle, use a fork or citrus reamer to juice the grilled citrus. There should be about 2 cups of grilled citrus juice.
Stir the remaining 1/2 cup sugar into the juice and add the tequila, stirring to combine. Refrigerate until chilled.
When ready to serve, pour the drink mix into a large pitcher of ice. Add the sliced citrus rounds and serve over ice in glasses rimmed with sugar.