Add Devise

devise setup.rb

- Add in Gem file
 gem devise

- Install devise
$ rails generate devise:install

- Generate user model and routes 
$ rails generate devise user

- Add extra fields
$ rails g migration addFieldsToUser name:string company:string address:string

- Generate views
$ rails generate devise:views users

- Add other modules
devise :database_authenticatable, :registerable, :confirmable,
         :recoverable, :rememberable, :trackable, :validatable

Model file should be like this

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :token_authenticatable, :encryptable, :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable, :confirmable,
         :recoverable, :rememberable, :trackable, :validatable

  # Setup accessible (or protected) attributes for your model
  attr_accessible :name, :email, :password, :password_confirmation, :remember_me
  
  validates_presence_of :name
end