Well, coming from someone who has organized these events, I can tell you that you'll put in a lot of hard work with a smaller than expected turnout.
Keep in mind, a lot of the watchers of the site are too young to travel to philly on thier own, and the older ones have jobs or college. Limiting it to just the philly area will be harder for a turnout, but in the long run a lot easier to organize.
What I'd recommend is finding a charity already happening and having newgrounds as one of the sponsors. That way the charity will get free advertising from you, and you'll get advertising from them. As well you both will be able to combine your "people" for a larger turn out.
The other option is something like a "party" weekend. Set a weekend, more than likely at least the summer, and post what weekend, make up a list of hotels and an agenda for the whole weekend. You might have to put a limit on the number of people coming however, unless you plan on renting a gathering hall.
Have any questions let me know, I'm a partner in a production/event company. I'd be happy to help.