Critters, one of my favorite workshopping sites, addresses this question:
Is sending through the group considered publication?
In a word: No. Editors recognize the utility of critique groups and that many authors belong to them. Being seen by a restricted set of other authors is not publication ("publication" means available to anyone, i.e., "the public").
I know for a fact of stories workshopped on Critters have gone on to be published by the biggest magazines in the field, so I'm inclined to believe 'em.
In general, a good rule of thumb is this: if you can find the actual text of the story using Google, e.g, by searching for the title, for the author name, or for a line or two from the text, then consider it published. If you can't, you're in the clear. So, for example:
- A forum which is locked behind any kind of password should be OK.
- Publicly sharing a link to a temporary document which you can take down after getting feedback should also be OK.
- Emailing text directly to a limited group of online acquaintances is fine.
- Posting your entire piece on a public forum - not OK.
- Posting your entire piece to your personal blog - not OK.
An additional crucial point is to check the terms and conditions of the site you are posting on. Some sites and forums claim ownership or the right to re-use any posted content (the Stack Exchange network, including Writers.SE, is one such site). If anyone besides yourself, the author, can claim ownership or re-use rights over your text, that can be a significant problem, and can conceivably render your work unpublishable.