This isn't the "company" party, there is going to be one of those too. This is a party at the GM's house. What does one wear to a "casual" party at a bosses boss party?
No matter what you choose to wear, you will still look great.
I went to WI for some corporate training for a resturant chain, one of the nights ended with a dinner at the Founder/Owner's house....it was a good time. I made myself at home & they didn't care. His daughter came home from a soccer game, covered in mud...when she came inside the HUGE house I was sitting at the dinning room table and I told her "come on in, are you hungry?" she was like...."this is my house and where are my parents"
They had a three legged dog, named Fudge. I was making jokes about the dog. I was one of the few people not drinking. They had a galley off the kitchen full of booze and wine. His wife cooked while he grilled out. Nice folks.
http://www.culvers.com/about/default.aspx