added the migration

db/migrate/20110126232040_add_unique_index_on_invitation_service_and_invitation_identifier_to_users.rb

+class AddUniqueIndexOnInvitationServiceAndInvitationIdentifierToUsers < ActiveRecord::Migration
+  def self.up
+    add_index(:users, [:invitation_service, :invitation_identifier], :unique => true)
+  end
+
+  def self.down
+    remove_index(:users, [:invitation_service, :invitation_identifier])
+  end
+end