I have had this fantasy on occasions, but it got put into reality once by my three girlfriends.
They took me to a drive-in movie and I sat in the back seat with this guy who knew who I was.
The other two had boyfriends and it was a bit cramped in the car.
I eventually sat on his lap and had my arm around him.
The others kissed and cuddled so we did a bit too.
His searching hands did get under my blouse and I responded in kind.
We giggled and joked about it as we were aware of what the others might be doing and they had set both of us up.
It was not unpleasant and we enjoyed the moment.
We thought if they want us to play girlfriend/boyfriend, why not!