[Zope-PAS] auto add users

[Brecht Samyn]

What is the best place (plugintype) to register a new user automaticaly when he
is authenticated?

I wrote an extraction, an authentication and a group plugin for our Shibboleth
environment. A user is valid if a Shibboleth attribute with a valid userid is
present in the request. Now I want a Shibboleth authenticated user to be
registered via the portal_registration tool automatically.

When I put the code in the authentication plugin, I get a permission denied
error because only authenticated users have "Add Portal Member" permission (and
I want to keep it that way). 

Any suggestion of what the best place is to put the code?

Brecht