This is only partially possible, as far as I am aware. There is no problem creating subdomains, and each subdomain can have a unique user, but I believe there is no way to have yourself as a "superuser" with access to everything.
Each user can manage one or more domains/subdomains, but each domain/subdomain can only be owned by a single user. Of course, if you are issuing the username and password combinations, you can simply log in with those.
Simon Jessey | Keystone Websites
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