01-schema.rb

ActiveRecord::Schema.define(:version => 20120330150918) do

  create_table "users", :force => true do |t|
    t.datetime "created_at",                              :null => false
    t.datetime "updated_at",                              :null => false
    t.string   "email"
    t.string   "password_digest"
    t.string   "invitation_token"
  end

end

02-routes.rb

AppName::Application.routes.draw do

  resource :invitation
  resource :rsvp
  resource :sign_in
  resource :sign_up

end

03-invitations_controller.rb

class InvitationsController < ApplicationController

  def create
    @user = User.find(params[:user_id])

    @user.update_attributes!(
      invitation_token:   SecureRandom.hex(8))

    UserMailer.invitation(@user).deliver

    flash[:success] = "#{@user.name} was invited!"
    redirect_to @user
  end

end

04-invitation.html.slim

doctype html
html
  head
  body
    h1
      | Hey #{@user.to_s}.
    p
      | You've been invited!.
    p
      | Click 
      = link_to "here", edit_rsvp_url(invitation_token: @user.invitation_token)
      |  to begin.

05-rsvps_controller.rb

class RsvpsController < ApplicationController

  def edit
    @user = User.find_by_invitation_token!(
      params[:invitation_token])
  end

  def update
    @user = User.find_by_invitation_token!(
      params[:invitation_token])

    @user.attributes = params[:user]
    @user.invitation_token = ""

    if @user.save
      sign_in @user
      flash[:success] = "Welcome!"
      redirect_to @user
    else
      render "edit"
    end
  end

end