1_form.rb

class RetrieverForm
  include ActiveModel::Conversion
  include ActiveModel::Validations

  validate :existance_of_email

  def initialize(attributes = {})
    @email = attributes[:email]
  end

  def existance_of_email
    unless Invitee.where(email: email}).exists?
      errors.add(:email, "doesn't exist")
    end
  end
end

2_controller.rb

class Controller
  def new
    @retriever = RetrieverForm.new(params[:retriever])
  end

  def create
    @retriever = RetrieverForm.new(params[:retriever])
    if @retriever.valid?
      render :woot
    else
      render :fuck
    end
  end
end

3_form.html.erb

<%= retriever.errors.messages if retriever.errors.any? %>
<%= form_for :retriever, url: retriver_path do |f| %>
  <%= f.text_field :email %>
  <%= f.submit %>
<% end %>