Thats what me and mate use to do with some bird use to do alot we use to take turns each or together. But if i was rubbing infront of her, i use to grab and on hold to his arse lol. If i was behind her i use to geope her tits over her bra and top whole i rubbed myself against her ass.
Hardly groping is he! Missed opportunity really, more slapping her tits more than anything. Just get behind her and get a good handful while grinding her ass.