Don't use deprecated method

app/controllers/signup_controller.rb

@@ -104,7 +104,6 @@ class SignupController < ApplicationController
       password: @user.password
     )
 
-    @invitation.update_attributes invited_user_id: @user.id,
+    @invitation.update! invited_user_id: @user.id, used_at: DateTime.now
-      used_at: DateTime.now
   end
 end