We just had DH's office party last night and I was in a similar boat. He has only been with the company since mid-November, so this is the first office party he's been to. It was held at a country club, and the invitations said "Business Casual". So, I sent him to the girl in the marketing department who organized the thing to ask about what the women usually wear (Business Casual is so much easier for men!)
She told him that she wasn't sure what she was going to wear - maybe a dress, maybe slacks.
I wore a short black skirt (just above the knew), black dressy sandals (I'm in Houston - it's 80 degrees), and a cute crinkly striped blouse in shades of red, with some silver thrown in to look festive. I thought I looked pretty nice, but we were slightly worried that I might be overdressed.
Marketing girl - she was wearing a taupe satin, strapless, tea-length dress with a black lace overlay. THAT IS NOT BUSINESS CASUAL!! Now, to be fair, there were lots of women there wearing all sorts of things, but most were dressed up more than I was. And being the "new guy's wife", I felt somewhat self-conscious. Oh well. There's always next year.