In a large resealable plastic bag, combine the first 8 ingredients, shaking well to mix.
Add the steaks; turn to coat with the marinade. Refrigerate at least 8 hours or overnight, until ready to cook, turning the bag occasionally.
When ready to cook, set the grill to medium heat. Remove steak from the refrigerator; snip a corner of the bag with scissors and drain the marinade into a small saucepan; set aside.
Place the steaks on the prepared grill and cook for 5 minutes on each side (for medium-rare) or until desired doneness.
Bring the marinade to a boil over medium-high heat; reduce the heat to low and simmer for 5 minutes; stir in the butter or margarine until melted and serve with the steak.