email.rb

class Email

  def initialize(address)
    @address = address
  end

  def address
    @address.strip
  end

  def valid?
    address.match(email_regex)
  end

  def to_s
    address
  end

  private

  def email_regex
    email_name_regex  = '[A-Z0-9_\.%\+\-\']+'
    domain_head_regex = '(?:[A-Z0-9\-]+\.)+'
    domain_tld_regex  = '(?:[A-Z]{2,4})'
    /\A#{email_name_regex}@#{domain_head_regex}#{domain_tld_regex}\z/i
  end

end

email_collection.rb

class EmailCollection
  include Enumerable
  delegate :each, :to => :emails

  def initialize(email_addresses)
    @email_addresses = email_addresses
  end
  
  def emails
    @emails ||= emails_from_source.map {|e| Email.new(e) }
  end
  
  def valid
    emails.select(&:valid?)
  end
  
  def invalid
    emails - valid
  end
  
  def valid?
    invalid_emails.any?
  end
  
  private
  
  def emails_from_source
    @email_addresses.split(/[,\n]+/).reject(&:blank?)
  end
  
end

invitation.rb

class Invitation < ActiveRecord::Base

  validate :email_addresses_are_valid

  def send_invitations
    parsed_emails.each do |email_address|
      InvitationMailer.invite(email_address, email_subject, email_body).deliver
    end
  end

  private

  def email_addresses
    @email_addresses ||= EmailCollection.new(email_addresses)
  end

  def email_addresses_are_valid
    if !email_addresses.valid?
      message  = email_addresses.invalid.join(', ') + ' '
      message << email_addresses.invalid.many? ? "are not valid email addresses" : "is not a valid email address"

      errors.add(:email_addresses, message)
    end
  end

end

This code is incorrect, but the sentiment is there.