I would guess that the space was about 70 degrees; slightly chillier than my 'at rest' comfort zone, but perfectly ok given the amount of activity I was doing. Just after handing my coat to the coat check ($1), I was chilly for a bit, as people were still coming in and bringing outside air with them, but that was no more than a few minutes.

Dress... to be comfortable, with the knowledge that you'll be on your feet for most of three hours, and moving around a lot. Some of the audience were wearing fancy-night-out clothes, but most weren't. I wouldn't wear anything with frills that might get caught on random objects.