Hello and Welcome to the Chrons!
I. Not immediately no. We have a Critiques forum where members can ask for feedback on their work, but only those with at least 30 counted posts can put their work up there. Once you have enough posts, though, then you can post your short stories there, though there is an upper limit of 1500 words per thread and we'd advise that you only post, say, the first 600-800 words to begin with.
2. As much editing as you can manage, especially as to spelling and punctuation. It's best to put something polished up as that means those critiquing it can concentrate on other things that need work rather than dealing with mistakes you might have caught yourself. But don't worry about it unduly, and you certainly don't need to get someone to edit it for you before posting.
3. Critiques can be forthright, but we always make allowances if we know that someone is new to writing or has eg dyslexia, or is not a native speaker, so just remind us about that as and when you post. And no one will be rude or offensive, because we simply don't allow that. So please don't worry.
4. We don't have any groups or teams or anything of the kind, but what tends to happen is that members gravitate to different areas of the site. So some of us spend most of our time in the Writing forums, while others spend a lot of time in eg Playrooms. What I suggest is that you look around and see the many different areas of the site and its sub-forums and just make yourself at home in the places that you find most congenial or interesting. And if you want, you can tell us a bit about yourself in Introductions where you can be welcomed officially!
https://www.sffchronicles.com/forum/introductions/
You're not being gauche, so don't worry. If you have any questions, just ask -- we're always happy to help!
Meanwhile, I'll move this thread over to our Writing Discussion thread, where it will get more notice. YA Fiction is really for already published books.