In a pitcher, measure and add the chilled pineapple juice, tequila and simple syrup. Stir to combine.
Fill glasses to the top with ice. Fill 3/4 of the way full with the pineapple mixture. Top off with beer leaving 1/2 and inch at the top of the glass.
Slowly pour 1 to 2 teaspoons (per glass) grenadine into the glass over the back of a spoon or by drizzling it down the side of the glass, allowing it to settle at the bottom of the drink.
Garnish with pineapple slices and cherries.
Notes
TO MAKE THE SIMPLE SYRUP:
Add 1 cup both granulated sugar and water to a small saucepan. Bring to a low boil and stir until the sugar has dissolved. Remove and let cool completely before making this drink. Best if made in advance and refrigerated.