Tend to agree for 4 people, a cab is certainly going to be competitive with a shuttle (although you will need to add tolls and tip, it is at least $46 total).
For fewer people, since the GH is right at Grand Central, airporter buses tend to be easy and there is always the Skytrain to the subway. Bear in mind that Skytrain to the subway really isn't that much cheaper for 4, it's $7 each (a bit less if it can becombined with a larger dollar value metro card purchase), but it is an option if time is much less important than money.