I live in LA, and my husband and I both have been in and around the entertainment industry for years. But I do not dress like most women in this town, so maybe I'm not the best person to answer this.
You will see women from 15 to 50 in super short mini skirts, sky high heels and skimpy tops. You will see a lot of spandex and glitter. You will look at the younger women and wonder how their mothers let them leave the house looking like that. You will look at the older women and think they might be trying a tad too hard.
You will also see lots of dark jeans and glitzy tops, as well as short dresses and bare legs.
You may also see some wild costumes. Latex. Leopard skin jumpsuits. Skin tight black pants with a leather vest. And that's on the guys!
Seriously - you will see anything and everything. If you're comfortable going short and sexy you will certainly fit in. If not, there will be lots of women in premium jeans and a really nice dressy top. (Expect it to be warm in the theatre - you probably won't need your wool sweater.) A leather jacket is always a good bet.
But it really all depends on your own taste and style. I always feel out of place at these events. I am not into showing skin or glitter at all, so I usually go for black and simple - black dress pants, a cami, a tailored jacket, and heels. I do have one pair of pants I call my "rock star" pants - fairly tight, black, in a jacquard fabric. But honestly - I am so far from anything rock, even when I wear those. I usually feel silly when I try to compete, so I just wear what I feel good in and sit back and watch everyone else.
But really - wear whatever makes you feel good. You can hardly go wrong. (Well, maybe if you wore an Amish dress and bonnet. But even then people would just think you were a performance artist.)
What are you thinking of wearing?