Assign non-form data to a new user on sign-up with Devise.

registrations_controller.rb

class RegistrationsController < Devise::RegistrationsController
  protected
    def build_resource(hash=nil)

      # let devise do it's magic with the form data.
      super

      # assign any additional information the form doesn't populate.
      # (e.g. @organization is populated in ApplicationController.)
      resource.organization = @organization

      # maintain the existing interface
      self.resource

    end
end

# Also, update routes.rb to utilize your new registrations controller.
#   devise_for :users, :controllers => {:registrations => "registrations"}