Iframes present their own problems, mainly with search indexing and accessiility issues.
Personally I'd set up a small Content Management System, that allows registered users to create a page which is added to the site navigation. You can control the page layout (template, CSS, SSI or whatever) and only supply editing rights for text or text and images.
I don't know of a simple way of doing it without getting into server side scripting.
What you want to be able to do requires writable access rights to your web server and as such, you'd really want a very robust login system and carefully locked down editing rights. Depending on your web server I'd be looking into PHP or ASP with an underlying database. Anything less is potentially more open to security issues.