<span data-id="42" class="js-join">Кликни!</span>
<div id="modal-forum-join" style="display:none">
  <div>Модальное окно</div>
  <button class="js-modal-join">Теперь кликни меня!</button>
</div>

span {
  text-decoration: underline;
}

$(document).ready(function() {
  $(".js-join").click(function() {
    var id = $(this).data("id");
    $.fancybox.open({
      src: "#modal-forum-join",
      opts: { 'modal-id': id }
    });
  });

  $(".js-modal-join").click(function(e) {
    var id = $.fancybox.getInstance().current.opts['modal-id'];
    console.log(id);
    alert(id);
  });
});