They wear whatever they're comfy in....generally slacks or shorts and a top...or barechested at times for the guys. It tends to depend on the gathering and stuff.
At NEST most people dress like any casual party, although I must say Jonmath always manages to wear something spectacular. His lovely lady Kittentoes was quite a sight to behold at this years NEST, resplendant in a lovely corsett.
I usually wear either bluejeans or shorts, along with a T-shirt. Many wear sandals or shoes that can be easily and quickly removed.
At Nest I wore loose capri pants and a T shirt...around the hotel I was wearing a sleeveless top 😉 and the same capris. If you're going to lee (or ler I suppose), loose comfortable clothing is the best. And natural fabrics like cotton are a must (the sweating factor). OH...and sandals with a velcro closure.
For more intimate gatherings, I've worn shorts and a sleeveless top or even a sports bra.