How much can you share unpublished forum articles? There are some journals, such as Behavioral and Brain Sciences, that publish a single target article per issue, followed by 20 to 30 commentaries on that article, and then a rebuttal/synthesis from the target article authors. Their rough workflow seems to be:


Authors submit a target article.
Target article is peer-reviewed as in a normal journal, with a focus on if it is of sufficient interest to warrant discussion in such a forum setting.
If accepted, the editors send a massive number of invitations to associates of the journal, soliciting commentary proposals on the target article. A dark-net link to the final version of the target article is included.
Authors submit commentary proposals, and the editors review them, selecting a subset that would make a good discussion.
Selected commentators write their articles and submit them to the editors
Editors do a quick review of commentaries and pass them on to the target article authors for a rebuttal.
Target article authors write a rebuttal.
Everything is published at the same time (this is the stage that BBS makes anything directly publicly available from their site).


At stage [3], the number of commentators contacted is significantly in excess of what you would for soliciting peer review. A link to the post-review paper that is accessible without any logins is also distributed, but this link is on the 'dark net' -- nothing on the main site links directly to it. The contacted associates are encouraged to share the article with their colleagues and bring non-contacted collaborators on board if they want to write a commentary. 

My question is: to what extent can the sharing in step [3] be taken?


Can you share the article privately as much as you want? I.e. can a non-associate that was sent the article by an associate (but not the editor) then forward it to another non-associate?
Can you contact the authors of the target article with questions or comments that make it obvious that you've read the article? (Note, that you wouldn't be able to do this for an article you are peer-reviewing)
Can you write a blog post about the article before it comes out, making a summary of it, and referencing it as (to appear)?
Can you share the link that was emailed to you publicly (say on your blog)?


Note that the target authors often have a preprint of the target article linked from their website, kind of like a white paper (although it isn't always exactly the same version as is sent out by BBS). I am interested in answers to the above questions in both cases: when a preprint is available from the authors and when only the BBS dark-net link exists.
If you are unclear on the matter, I would ask the person who sent you the article. Asking the editor or authors may get the person who sent it to you in trouble. In most cases, the first answer is probably no. However, it may be true in some cases where there are no provisions on the confidentiality of the article. When sent out to be peer reviewed, they usually stamp the title page of the article with the confidentiality clauses. I don't think you should contact the authors until the peer-reviewing process is over, but that doesn't mean you can't. They might just refuse to comment. The last two questions you have posted... You most definitely cannot do that. You could get in major trouble for early distribution of research, especially if the project is not finished. You may face academic dis-accreditation which would make it very difficult for you to ever be involved in the academic community. This would be especially true if you wanted to peer review articles for that journal in the future or if you wish to publish articles with them. You would likely also face difficulties publishing in related journals, which may be problematic if you choose to work in an academic setting. If the author has a link to the article on their webpage, I would suggest you contact them and ask for their permission to post it on your blog. Doing so without their permission is an infringement on their property.