I don't have the type of shell that was initially described, at least I no longer have it. My own shell is more of a privacy thing. You know : a line in my mind over what type of tones, words, emotions, issues, things I'm comfortable expressing with xxx in xxx situation. The usual, but I'll be using the first described one in the following:
I started initially coming out of my shell after joining a small theatre group around 12 y/o. I'd had some bullying & friend issues etc. before that which had made me withdraw into my shell some more. The theatre group was a warm and comfy setting in which I didn't feel pressured into anything (well except the occational group performances), so I started opening up slowly and engaging more. Chatting with people, expressing ideas. I managed to form friendships with three people from the group that have lasted about 8 years now.
The second thing that affected me was when I got a job from a supermarket cashier/clerk, part time for a 1½ and full time for a year. That was extremely taxing, no private space (except inside my head), new people all the time, blah. But I've noticed it's helped me in dealing with complete strangers and iniating, and also bossing people around.
The funny thing is, I have a cousin & her mother who are very extroverted and view introverson as a bad thing, still pressure me into more activity or whatever and giving advice on where I should go, what I should say, what I should say. Not helping. Idiots, good meaning idiots. After that I normally need a while to clear up my head out of all the stuff they've been feeding me, the standards etc. I normally go walking alone to the downtown, and start feeling better after that. Aka braking their advice right away.