Interesting point. I went back to your original post and noticed that you asked about two passengers. Almost nobody picked up on that, but it's important. Car services and taxis allow two to travel as cheaply as one, but that's obviously not the case for buses and subways. Two express buses from JFK to Grand Central would be $26, very close to a taxi (max $40, with generous tip). The taxi will leave immediately and take you right to your destination.

Bruce