Guaranteed it will be outside! There are not, as far as I remember from last year, any indoor venues at Keystone that can support this size and type of even. Likely Keystone Village. Go as formal as you like, but if you wear heels, carry comfortable flats in your bag, or even tennies, and a light jacket of some kind. I guess a mink stole might work, but...........................
The email did also mention that some various accoutrements would be available, and I am thinking along the lines of a clip-on bow ties for men, and god-only-knows what for women.
Me, I intend to go fully formal and wear my oldest InstCon T-shirt, oldest InstCon jacket, and clip something on as a disguise. However, every InstCon has had a theme, and every year a significant number of folks dress extravagantly in support of that theme for the reception. It really is entirely up to you! I love costumes, and would go whole-hog for this, but I really like to travel very light and save some room for swag. hate airlines charging me extra, hate having to go to baggage claim, and all that jazz!