Sorry, work scheduling has been really screwing with me. Late night some days and early mornings others. I do need more info though. Is this the first time this portal has been activated? You said it was NYC in Marvel; but where exactly? Did Spider-Man just step through expecting full well what was happening? Was the portal up high, showing the city skyline and Spider-Man stumbled upon it while slinging?
Do you have a set plan; or did you want me to think up those details?
EDIT:
I just went ahead and filled in some of the blanks for you. In a lot of the co-op stories I've seen; they've used Stark's building to gather. It allows a good view of the city; and it fits into the story building here.
EDIT2:
Here's what my Spider-Man looks like.
Just no extra red webbing; all dark-as-night blue, the red spider, white eyes, and of course sporting six arms.