Do not miss this fantastic post detailing how to go about lazy user registration for rails apps and the example provided is based on the ever popular Devise — the concept is clean and well explained enough to be adapted to any auth system of your choice.
As an addendum, that may not be clear right off the bat, is the User model should have
class User < ActiveRecord::Base
ACCESSIBLE_ATTRS = [:name, :email, ...]
attr_accessible *ACCESSIBLE_ATTRS
end
That's why the AnonymousUser lists the User's attributes as accessible albeit as the 'registrant' role.