I prefer to sit if given the option, but I do whatever everyone else is doing when I'm in a group. So if they are standing, I will stand, and if they are sitting, I will sit, even if one or the other would be more comfortable for me at the time. However, I do the same thing about just listening to everyone else and rarely taking part, even when I have things I want to say. It's hard to know when to jump in, and whether what I have to say is actually interesting/relevant or not. I also feel very uncomfortable and "on guard" in groups, and I focus on doing my best not to seem weird or anything, which usually means not saying a word and seeming weird anyway.
Edit: Thinking more about it, standing is kind of hard for me. I'm perfectly fine with walking, but when standing still, I feel like I have to make a conscious effort to keep my muscles tensed up right. I tend to lean on things if I can, or I shift my weight back and forth from side to side, like sideways rocking, if I have to stand still for long, which my Dad has always teased me about (he calls it "dancing", and I probably wouldn't know I did it if he didn't always tell me to stop dancing). Not really sure why, but it's much more comfortable for me to sit down and let my muscles relax than get all tensed up standing still.