I vary between totally silent and occassionally very gregarious. The joys of having 4 and 9, and conversely 7 in your tritype!
1. There's no consistent trend. When meeting new people in some kind of environment where I know we share a common interest, like on a music or politics event, then I'm okay. But randomly meeting new people, unless I feel responsible to, is not going to see me at my most overt.
2. I'm loudest when I'm with close friends that understand me

but I still might go quiet for a few minutes then rejoin the world of the living.
3. Pretty quiet around my family usually. Too scared of causing conflict, too many values being crushed. It's been okay for the last few days but generally I avoid my family. And then the extended family gatherings, or when we have family friends over... I just go completely quiet because they're all sensors and they all don't give a shit about anything I'm interested in. Not that they're not good people, because they are - but they have very different concerns to me in life, and are very different people to me.
4. At parties, it really does vary too. A small gathering (less than 20) with people I really like is absolutely fine for me to manage, and I'll be really Ne fueled and witty at times and take a step back when I need to recharge a little. But bigger gatherings where the music isn't very good (a.k.a. the music is wildly popular in the last 3 years), where I'm not comfortable with all the people there, are horrible. I either spend all my time looking after people and get incredibly exhausted very very quickly, or just retreat inside my head and find a guitar and escape from everyone for several hours. No one realises I was gone until I get back.