I've never given a thought to dress at our guild. I either wear what I wore to work or I change into jeans and a t-shirt. No one is dressy, but if people came from work they might be business casual.
Even our presenters aren't often dressed up. I think if you came dressed to the nines (an evening gown might get some looks) or in pajama pants, no one would really think otherwise- everyone just wears what they want.