Combine hoisin sauce, beer, ginger, garlic, soy sauce, and brown sugar in a large bowl. Remove 1/2 cup of this mixture and reserve. Add chicken thighs to bowl, turn to coat. Cover and refrigerate, turning occasionally, at least 2 hours or over night.
Prepare grill for direct cooking.
Remove chicken from hoisin mixture and place, skin side down, on lightly oiled grate over medium-high heat. Grill 10 minutes; turn and brush with 1/3 of reserved hoisin mixture. Grill an additional 10 minutes; turn again and brush with another 1/3 of the hoisin mixture. Grill 5 minutes more and turn chicken one last time and brush with remaining hoisin mix. Grill 4 to 6 minutes longer until cooked through and instant read thermometer reads 170°F at thickest part of thigh.