group.rb

class Group < ActiveRecord::Base
  include ActiveDirectory

  has_and_belongs_to_many :users
  
  def members
    @members ||= ldap_members(name)
  end
end

module.rb

module ActiveDirectory
  def ldap_authenticate(user_id, password)
  ...
  end

  def ldap_members(group_name)
  ...
  end

  def ldap_remove_member(user_id)
  ...
  end

  def ldap_add_member(user_id)
  ...
  end
end