There is a GPS widget imbedded in your bib that's connected to you registration number and allows your friends to track your position using the marathon app. However, don't count on this too much—not because it doesn't always work, but because, with so many people in the city using it, it has a tendency to crash. Where you want people, and what you'd like them to have for you, is up to you.
However, my general advice on this is twofold. 1. Know, generally, where they plan to be: some parts of the route are more crowded than others, and this will make it easier to look for them. And 2. If it's a matter of choice, it's more important to have people near the end than the beginning. I had two (very loyal and dedicated) friends waiting for me at mile 22 in Marcus Garvey Park (also known as the official address of the dark night of the soul) and knowing they would be there was pretty much the only thing that kept me going from mile 18 onward. It sounds like a little thing now, especially if this is your first marathon, but in my hazy memory of that long, long run last year, what stands out most clearly is how utterly dependent I became on seeing those faces (and that carton of chocolate milk!) in Harlem.
Also, beware the finish line. Once the route turns off Central Park South and back into the park itself, it becomes difficult to spot people (and for people to find spots). Everyone has visions of their loved ones waiting for them at the finish line, but the reality is the closer you are to the end, the harder it is to spot anyone. Better to have them somewhere between miles 22 and 25 when utter, total exhaustion truly kicks in.
A note on the final stretch: Right now, mile 25 probably sounds like the end of the race. Hell, only 1.2 miles to go! Homestretch! No. Not at all. The home stretch is the last 100 feet. A better analogy: the Holland Tunnel is technically 1.6 miles long, except in rush hour when it is never-f***king-ending. That's what the last few miles of the marathon are like: New York in rush hour traffic. Actual distance ceases to have meaning. Just be prepared to be very tired. Don’t lose heart.