as of this FAQ...http://iwebfaq.org/site/iWeb_Group_website.html
There's no way to edit an iWeb website from a PC
iWeb is really bad for sharing a website management since both guys/girls need an Apple computer and the same Domain.sites2 file. I once had a visitor who had a company all using Apple, they did put the domain.sites2 file on a USB stick and if someone wanted to update the website they just inserted the usb stick and that's it. For them it worked since they all used Apple and were working at the same place. As soon as there's a distance (say home-working place) iWeb gets a mess.
I don't think that's a solution for you.
In other words: iWeb is only made for one-person website management.
If you want to share the website's management you should think of using something that is web-based meaning that there's nothing on your computer it's all online and which also means that you can edit the website from no matter what computer on planet earth (and outside of planet earth as well if you have satellite-internet) without any problem.
I think the most used online-website-builders are
Building a website online has another advantage too: no fear about overwriting with an older version.
I never tried Joomla but the websites built with it look great and they say it's easy to use as well. They also have a great and vast community (if you need help/support/add-ons, ...)
Wordpress simply works.
Most online business have shared website-editing but of course it depends on the size... Someone selling his own home-made-crafts usually does it on his own.