Quoted:
I own a mostly stock Stag Arms Model T2 (added PA red dot and foregrip).
I got a job offer in Manhattan...
How to I make my AR NY state legal?
Do I have to do anything involving law enforcement before or after I move (registering my weapon, etc)?
First off, congratulations on the job offer, however, if you already have a job where you live in RI, consider the following. Manahttan is one of the most expensive places in the world to live, probably just after Hong Kong and London. It's a little cheaper if you're willing to commute, however plan to spend no less than 1 hour, and more likely 1.5 to 2 each day, each way if you work peak hours if you live outside the other boroughs, or Jersey City/Hoboken. The extra money that out of towners think that they'll make by taking a job in Manhattan usually gets eaten up rather quickly by these things, along with the ridiculous parking fees, tolls, and city income tax.
Now, if you still want to move, realize that while living in Manhattan if you work there affords the most free time/least hassle not spent commuting, it is not without it's drawbacks. The city does have it's own income tax, all guns are illegal in the city, you may get a license to posses certain long guns, and most handguns, but it's not easy, and costs time, money, and aggrivation. You'll find it difficult to live like most middle class families in the rest of the country while living in Manhattan unless you make $250k and up per year, and thats living in an ok neighborhood, maybe having 1 car for yourself and your wife, your kids will most likely have to go to private school, and you'll want to escape on the weekends. Living outside the city will give you some relief, but again, you'll have to commute. If you work in lower Manhattan, New Jersey actually presents the best choice as far as commuting, as it's the closest, but then you're dealing with NJ gunlaws, which aren't nearly as bad as NYC, but there are some hoops to jump through.
Good luck