Thanks for your time and reply Mike.
Although this portal is an update, I don't think that is related to the issue.

Old portal 3.3.5 updated to 4.8.2.
Installed 1.0.0.4 ad authentication
User ad authenticated onto site (possibly using old authentication) when site was 3.3.5.
During creation of portal I enter domain\username and bogus password and portal creation fails with error message "username already exists for the user specified"

Previously I was able to specify the administrator ad username and a bogus password.  Portal would be created and when user auto-authenticated, user had site admin role.  This was an advantage as asking the user to log in and then assigning the user account is a step saved.  Selection of a user account from all portals would be ok but cumbersome for portals with over 200 children.  I cannot think of a scenario where needing the portal to have a "new" account created is a advantage.  Leaving the password blank and allowing only ad (other) auth to write seems best.

I am going to hunt for the point in the source and code out today if I can.

Thanks for any pointers/assistance.

Mike Horton wrote Can you post the steps to recreate this problem? I just tried to register a user that belongs in one portal on another without any problems(*). I also created a new portal with that user as the administrator without any problems. * My first attempt to register the user did fail with the error that everyone is getting but that's because I used the wrong password. The user has to use the same password across portals and, to my knowledge, this has always been the case in DNN.

I created a user with the same username, password, and e-mail address across portals. I got the verification e-mail, but after I entered the verification code, I get this error message:

The E-mail supplied is invalid.

I also logged in as admin and see the user registered as an unauthorized user. I tried to authorize the user, but I got the same error message. Note that I am using the default membership provider with RequiresUniqueEmail set to true in the web.config file.