From my past experience with Otakon, it never ever hurts to plan way ahead of time, especially in a large cosplay group! I've coordinated several group shoots in the past at Otakon (notably Vocaloid). Feel free to contact me through here, or via my page (Soul-Drive Photography, in the signature).
p.s. There's a crescent moon on Friday and Saturday night at Otakon, just sayin'. ;D
2013 - AnimeNEXT, Otakon
2014 - PAX East
2014 - Otakuthon