Are you saying you want some to be able to edit their page, (http://egdomain.com/bla.html) from, your page (http://othereg.com/edit.html)
because there is only two possible ways you would be able to do that:
one is for them to give everybody full permissions on all their folders, and work out some way to access that,
or two is two set up a ftp thing, and get them to give you their ftp username and password details.