Require valid invitation to start sign-up process

config/routes.rb

@@ -4,10 +4,12 @@ Rails.application.routes.draw do
   get 'welcome', to: 'welcome#index'
   get 'check_your_email', to: 'welcome#check_your_email'
 
+  get 'signup', to: 'signup#index'
+
   get 'settings', to: 'settings#index'
   post 'settings_reset_password', to: 'settings#reset_password'
 
-  resources :invitations, only: ['index', 'create', 'destroy']
+  resources :invitations, only: ['index', 'show', 'create', 'destroy']
 
   namespace :admin do
     root to: 'dashboard#index'