Does Jane hold the copyright to that story? Yup. Publication isn't necessary. Jane wrote it; Jane has the copyright. Showing it (or not showing it) to friends does not change its copyright: Jane has it.
Sharon thinks the story would be better if the main characters did not get married at the end of the story. Deanna agrees. Jane rewrites the story from the middle to the end, giving one character a motive not to marry the other.
Does Jane still hold copyright? Yup. She wrote it. Sharon's and Deanna's response to the story does not negate Jane's copyright even if she does something they suggest.
Jane sends the story to market. Back comes an acceptance letter saying the editor wants it, please sign this contract. Jane is so thrilled to have a sale that she signs without reading the contract.
Does Jane still hold copyright? It depends on the contract she signs. If she agreed to "work for hire" terms, the copyright now belongs to the buyer. (Many writers and inventors work for corporations that impose "work for hire" terms; the corporation owns the copyrights and patents for anything written or invented on company time--and sometimes, by that individual while employed there, whether it's done at work or at home.) But if she sold only "North American serial rights"...then she still has the copyright. That's why contracts must be read carefully, phrase by intricate phrase. When it comes to defending copyright against infringement, courts will look at the publishing contract (for contractual infringements) and the lack of contract (for other types of infringement.)
Rights that writers retain can then be sold separately to other buyers (for example, foreign language rights--maybe each additional country/language brings in only a small amount, but if you have a mere $500 advance from 10 countries, that's $5000. ) Even when the primary publisher insists on acquiring secondary rights (book club rights, audio books, etc.) negotiation will get the writer a better percentage of the take.
So...Jane had better read the contract.
Jane wrote her story on a computer....and she sends a copy of the story by email to another friend, Anna. Anna thinks it's a great story and wants to share it. Anna doesn't ask Jane...she just emails the story to five or six friends, who email it on to their friends, and eventually one of the recipients posts it on his website...without asking Jane's permission or even giving her credit for the story. That recipient's child, stuck on a homework assignment, plagiarizes Jane's story, turning it in two days later.
Now what? Much will depend on whether the story got to that website before or after Jane sold the story. Jane had a right to email a copy of the story to Anna. Anna did not have the right to pass it on without Jane's permission. Jane does not know that the story--which is due to appear in an anthology realsoonnow--is now loose on the internet. Several bad things can happen. The editor has sent page proofs of the anthology out to several proofreaders...and the editor or one of the proofreaders happens to have seen that website on which the story--without Jane's name attached--was posted. If the story was posted to the website before the date on which the editor first saw Jane's submission (if, say, Jane sent the file to Anna six months before she sent it to the editor, and it appeared on the website 5 months before submission) Jane will almost certainly be suspected of plagiarism. After all, the story was out there on someone else's website (someone living three states away, or in another country) without Jane's name attached, and Jane is a first-sale writer--she doesn't have a track record. Jane could have just copied it and submitted it as her own. Jane will have to be able to prove that she wrote it before it appeared on the website. If the story was posted to the website after Jane signed the contract, suspicion will be less, but not entirely eliminated. The student who turned in Jane's story to the creative writing class could try to claim that Jane copied his story--that he's the real author.
If the anthology is published before the story shows up on that website, Jane will be in the clear, but the website owner will be in violation of the law, because the website owner has published a work without permission of the copyright owner.
There's another bad possibility: Jane sends the story to Anna six months before sending it to an editor...and someone plagiarizes it, sends it to an editor, and it's purchased as that person's creation. Jane doesn't know about this (she doesn't even know it's on a website somewhere), but when her submission arrives at the market, it's immediately seen as a plagiarism of the story that's already been purchased.
Jane will learn to be cautious about what she sends friends in email--some of them are thoughtless/careless about sharing files. Editors don't steal stories from unpublished writers, but other unpublished writers certainly do. One would hope that Jane explains to Anna just how much trouble her unauthorized file-sharing caused Jane and that Anna never does that again. One would hope that the person whose site the story was posted on will take it down when Jane asks that to be done. (One would hope that all site owners made sure they had permission from the original author before posting that author's work, and that they always listed the correct attribution: The fact that something is posted on another site does NOT mean you have the right to post it on yours. Always ask the author directly.)