I imagine they do have Ribena in the U.S., as we have it here in Canada. If it's not in the regular juice aisle it might be in the "fancy" beverages aisle, next to the Orangina and energy drinks and stuff. It's definitely not a big thing or a major player in the market here though.
I think there probably are a lot of grape flavoured products in North America. If you think of any packaged candy, kids' "fruit" snack, beverage, or syrupy medicine, they all come in grape. But fruit punch, lemon-lime, and "wildberry", whatever that is, are just as common, I think.