To clarify, when you say "anyone", there are many qualifiers, no?
First, you have to go through a national member organization, and these, in turn, dictate the "technical committees" that are involved with development of specific standards. Do I have this correct?
The bottom-line is that, it seems (correct me if I'm wrong), to be involved in the development of a specific standard that you're interested in is not a simple matter for the vast majority of small-to-medium sized manufacturers.
I agree that being part of the ISO WG is not an easy task for most organizations.
However, you don't need to be a part of the ISO WG to be part of the development. The WG mostly drafts the text and revised it based on comments, but the comments (after the WD stage) are created by the National Committees mirror committees. So, if you are a member of your country NC mirror committee, you will receive the drafts and can comment. Membership of a NC mirror committee is usually open to interested parties, or they may have some kind of qualifier.