Devise Simple Invitation Code

new.html.erb

<% # in your custom app/views/users/registrations/new.html.erb %>

<div class='form-group'>
  <%= f.label :invite_code %><br />
  <%= text_field_tag "user[invite_code]" %>
</div>

registrations_controller.rb

# in app/controllers/users/registrations_controller.rb
before_action :permit_invite_code

def permit_invite_code
  devise_parameter_sanitizer.for(:sign_up) << :invite_code
end

user.rb

# in app/models/user.rb
attr_accessor :invite_code
validate :invite_code_valid, :on => :create

def invite_code_valid
  unless self.invite_code == "somecoolcode"
    self.errors.add(:invite_code, "invalid.  Ask Steve or try again.")
  end
end