How would I cite multiple authors from an online forum?
Starting with the principle that the goal of a citation is to make it as simple as possible for somebody to find the particular reference, I can see two ways to approach this publication:


Treat it like a journal special issue or chapters in a book, where there are multiple articles bundled together under one editorial title.
Treat it like a single multi-author article.


In this particular case that you have linked, it seems that none of the contributors have titles for their individual sections.  That means that you can't readily separate out the individual contributions.  I would thus recommend citing it as:


  Alice Waters, Jim Hightower, Eric Schlosser, [etc], "One Thing to Do About Food: A Forum." The Nation, 11 Sept. 2006. Web. 01 Oct. 2014.


Then you simply to who you are quoting in the main text like you are already doing.
If they had individual article subtitles, you could do:


  Singer, P, "Singer's Article" in "One Thing to Do About Food: A Forum." ed. by Alice Waters, The Nation. 11 Sept. 2006. Web. 01 Oct. 2014.


Without the secondary titles, though, listing Alice Waters as editor makes it sound like she's the editor of The Nation not the forum.

[Note: I think my style may be slightly off from your required format - the key point is the person/title pairings]