CUT peppers into quarters; place in large bowl. Add mushrooms and 1/4 cup of the dressing; toss to coat. Meanwhile, preheat grill to medium heat.
REMOVE vegetables from dressing; discard dressing. Grill vegetables 8 to 10 min. or until crisp-tender, turning and brushing occasionally with 1/4 cup of the remaining dressing. Slice mushrooms; place in bowl.
ADD peppers and remaining 1/4 cup dressing; toss to coat.