Hi Guys,
I think Ballgown would be a little over the top
Apart from that you will see a wide range of outfits, usually from the ladies, ranging from outrageously sexy and revealing, to classy and elegant.
It's not a ball - it's a social meet, starting in a hotel bar and progressing to a nightclub, so if in doubt I'd lean towards the nightclub dress code.
Hope this helps
Suds