In a mixing bowl, whisk together cilantro, pepper, garlic powder, salt, margarita mix, sugar and olive oil. Mix well.
Pour over chicken in a resealable plastic bag or bowl. Marinate 2 hours.
Preheat a grill to medium-high heat.
Cook the chicken on a lightly oiled grate until cooked through to an internal temperature of 165 degrees F, which should take about 7 minutes per side.