Ludum Dare is a popular game making competition where contestants have a weekend to build a game around the announced theme. The prize is your product.
The competition usually starts around 9 p.m. EST on the Friday of the compo weekend. I'm not entirely sure when the best time of day to meetup and do this would be. Maybe we could have a two day event starting mid-morning on both Saturday and Sunday that goes on until everyone leaves.
The 48 hour competition is solo, but there's an extended 72 hour competition that allows multiple participants, so people in the group could team up if they wanted to.
As for the where, I have no idea.