The app is called Spireworks. Someone with the app has to invite you with a link. This used to be easier to find/buy because the app used to have 5 invites per user. Now its only 1.

I used to occasionally sell these to people in person for about $100 and then on the 5th one I would send the invite to myself and get 5 more and repeat the process.

There are probably people out there still selling invites for between $30-$100 though.

Once you are on the app, you have the choice of at least 2 buildings. The BOA tower on 42nd and 6th and the H&M tower in times square. You can only access them from sundown to midnight.

You can do a variety of things with the lights on the spires like change colors, directions, pulsate etc. You only get 2 minutes at a time and there can be 1 or 2 other people on it too. Thats why they can go a bit crazy sometimes. If there's more than 2 people then there is a queue formed. You can do this unlimited times in the evening. It does get boring after a while but can be a cool party trick or to impress a date (what's your favorite color? then change it to that immediately).

In recent years, some nights are blocked off to raise money for charity, so you have to pay like $10 or $20 to be able to use it that evening.

I haven't used it in years but pretty sure I still have access to it. I believe they had plans to add 1 WTC and the ESB at some point but not sure.

Edit: I don't have any spireworks invitations to give out or sell. You guys are on your own so good luck