twlist_builder.rb

require 'twitter'

# twitter app keys from dev.twitter.com
KEY = ''
SECRET = ''
TOKEN = ''
TOKEN_SECRET = ''

# Add members in batch
def add_list_members(list_slug, members)
  client = Twitter::REST::Client.new do |config|
    config.consumer_key        = KEY
    config.consumer_secret     = SECRET
    config.access_token        = TOKEN
    config.access_token_secret = TOKEN_SECRET
  end

  # get current members of the list to be more efficient about adding
  # however this is not necessariliy a cheap call, as it pages uner
  # the hood.
  current_members = client.list_members(list_slug)
  current_screen_names = current_members.to_a.collect{|u| u.screen_name}

  # remve anybody already on the list
  members_to_add = members - current_screen_names

  puts "----------- adding users to list -----------"
  puts members_to_add.inspect
  members_to_add.each_slice(20) do |slice|
    list = client.add_list_members(list_slug, slice)
    sleep rand 10
  end
end

members = []
list_slug = "slug of your list"

# Pull twitter screen names from source here and populate members array

add_list_members(list_slug, members)