Yes and no.
YES, you could provide a button that will let them upload files *somewhere.* But unless you have full control of the server on which you are hosting your web site (pretty unlikely), then NO they can't upload files to your server.
So this problem is tackled in two parts:
1. Setting up an "uploadable" space somewhere (FTP server, email, Rapidshare et al, MobileMe iDisk Public Folder, etc)
2. Creating a bit of HTML or Javascript that results in a button that initiates the upload procedure.
In short, this is really not something an iWeb site is cut out for, though that doesn't mean its impossible. Still, you may want to investigate something like Wordpress that makes this procedure a lot easier.