I love when players set up and run their own mini-events.

That said, Robinson Park is an illustration of how an open invite can go poorly as opposed to a more closed list of who is participating. I don't think either is wrong, its just good to try and avoid becoming too insular. 

The nice thing about both New York as a setting and the Fire Trolls as a plot element is they both scale very easily, so whether its just Arthur and a Martian suffering heat stroke or the entirety of the Justice League, S.H.I.E.L.D., and the greatest heroes of all time -- MID-WEST AVENGERS -- I can adjust to accommodate.

Not every scenario lends itself to that kind of creative wiggle-room however.