For what it's worth... editor is a role... have users log on to the portal with thier own identity, and assign them to an editor role which lives in each portal.

Separately, the editor user would be the same user across all portals, although her profile information would be different in each portal.

I am pretty sure if you create the user in the sub portal with the same email and password, it will essentially just add the existing admin user into that sub portal with the appropriate roles in that sub portal. There really is no difference between a portal and a subportal other than the alias used to access that sub portal and the fact that a folder/file must be created for that to redirect back to the root dnn site with the portal alias.

Getting back to the user question, the other way would be to purchase one of the several user management modules off of snow covered. I used one for DNN 2.x which allowed me to copy users from portal to portal and had all sorts of other time saving features.

Again, there is but one users table. There are a bunch of threads on this topic already which is why no one more knowledgeable has replied yet.