Quick and dirty add to MailChimp

require 'hominid'
mc = Hominid::Base.new({:api_key => 'you-api-key-goes-here'})
list = mc.find_list_by_name('Name of Your Subscriber List')
users = User.all
users.each {|u| mc.subscribe(list['id'], u.email, {:FNAME => u.first, :LNAME => u.last}) }