Many websites refer to people who sign up as 'users', which may be considered a bit derogatory; these people also participate, they don't just use the site.
In Russian, MediaWiki (the software that powers Wikipedia, Wikinews, Commons and sister projects) calls these people 'Участник', which literally is 'one who participates' [as a website user, and as a writer, as a reader, as whatever else you like; not as formal, active, or involved as 'participant' term expects]. What English language term is equivalent?