Can GitHub be used for writing a collaborative article?

You certainly could, but I don't know if it's the best choice. A couple of questions come to mind. Is this a text-based document format or are you planning on doing your writing in something like MS Word? If the former then I think it could work well. If the latter I would say it may be less effective.

What about your other collaborators? Are they savvy enough to use a DVCS? That would have some influence as well. I don't know how strongly you need the document versioned, but I could see using git as overkill.

I've found that using Google Docs works well and has a revision history, although it's obviously not as robust as would be found in a VCS.

I think it would work great. The Ruby on Rails guides are on a publicly write/readable repository at GitHub, for instance. You get get Git things for free (branches, blame, general version control features), plus you'll have a reliable backup and publishing mechanism if you like.

Given that the contributers are computer literate enough to successfully use Git, that is.

If you write it in Markdown, you can throw inline HTML into it (just by itself like you can do on Stack Overflow). Easy to write, easy to style, etc.