Foo.html

<% with $CommentsForm %>
  <% if $IncludeFormTag %>
  <form $AttributesHTML>
  <% end_if %>
    <% if $Message %>
    <p id="{$FormName}_error" class="message $MessageType">$Message</p>
    <% else %>
    <p id="{$FormName}_error" class="message $MessageType" style="display: none"></p>
    <% end_if %>

    <fieldset>
      <% if $Legend %><legend>$Legend</legend><% end_if %>
        
        
        
        
        {$Fields.FieldByName('FieldGroup1').FieldHolder}
        
        {$Fields.FieldByName('FieldGroup2').FieldHolder}
        
        
        
        
        
      <div class="clear"><!-- --></div>
    </fieldset>

    <% if $Actions %>
    <div class="Actions">
      <% loop $Actions %>
        $Field
      <% end_loop %>
    </div>
    <% end_if %>
  <% if $IncludeFormTag %>
  </form>
  <% end_if %>
<% end_with %>

foo.php

<?php

$fieldGroup1 = CompositeField::create(
  TextField::create($this->Intro_Name),
  TextField::create($this->Intro_Something)
)->setName('FieldGroup1');

$fieldGroup2 = CompositeField::create(
  TextField::create($this->Intro_Email),
  TextField::create($this->Intro_SomethingElse)
)->setName('FieldGroup1');

$fields = FieldList::create($fieldGroup1, $fieldGroup2);

$form = Form::create(...