I get not wanting to haul food a long distance. Take keto snacks for the drive so you don’t get there and feel you could eat anything put in front of you.
If they have green beans almandine instead of the casserole thing, was never a fan of that dish, it whould be a good choice. If they have broccoli with cheese sauce on offer that could work. Asparagus if simply steamed or sauted with butter. Even if someone made one the the veggie dishes with a seed oil, one meal with friends, shouldn’t be a worry. Do your friends/family eat mushrooms stuffed with carb meat? Deviled eggs should be fine too. Turkey and ham should be fine. A glass of white wine would work too.
And if you really want to avoid people thinking you aren’t eating anything, take a small serve of a couple of things and move them around your plate. Enjoy the company.
You might want to have a chat with your hosts to see what they are expecting everyone to bring? So you can go prepared? If you are just starting out with keto, you will want to be very cautious, if you have become fat adapted, a bit of something with small amounts of carb like breadcrumbs, you should do ok.