The osmfoundation.org website can only be edited by those with an editing user account set up on this website. This is not a shared set-up of user accounts with any other website. See the list of user accounts.
The Communication Working Group are the primary editors of this website, but user accounts have been set up for many other people involved in the foundation working groups. This has happened on a per request basis, with (admin users being the ones who can create new accounts)
Who should be an osmfoundation.org website editor?
Broadly speaking, anyone who is active on any working groups can have an account. This is partly because we are using (and encourage using) the site to house minutes of meetings.
These trusted folks are also able to edit the Main Page and other important parts of the website. The idea is not really to throw this open to the chaos of wiki editing by lots of people (the choice of MediaWiki as a CMS for the site is not a reflection of any intention to be openly editable) We have an OpenStreetMap wiki for that!
Important pages would ideally be carefully written or at least reviewed by the Communication Working Group, and so if you want to see changes, it's a good idea to contact CWG about these, even if you have access to edit here.
CWG is also in charge of this policy. If you want to suggest a change of approach for editing the foundation website, please feel free to contact CWG or perhaps even join CWG to work on such things.