Combine the sugar and 1/4 cup water in a small saucepan. Bring to a simmer, stirring to dissolve the sugar. Chill the simple syrup until cold. Meanwhile, chill 6 rocks glasses in the freezer.
In a large pitcher, stir together the bourbon, lemon juice, bitters and the chilled simple syrup. Add the beer and stir.
To serve, pour the cocktail into the chilled glasses over ice. Top off with a splash of seltzer, and garnish with lemon twists.