user_signup.rb

class UserSignup
  def initialize(params)
    @params  = params
    @success = false
    signup
  end

  def user
    @user
  end

  def success?
    @success
  end

  private

  def signup
    @user = User.new @params[:user]
    if @user.save
      @success = true
      # This is where you start everything once a user has been
      # successfully signup.
      # 1. send signup email
      # 2. run callbacks
      # 3. run background processes
    else
      # do nothing
    end
  end
end

users_controller.rb

class UsersController < ApplicationController
  def create
    user_signup = UserSignup.new(params)

    if user_signup.success?
      redirect_to user_signup.user
    else
      render 'new'
    end
  end
end