<?php if($this->item->params->get('itemExtraFields') && count($this->item->extra_fields)): ?>
      <!-- Item extra fields -->
      <div class="k2ExtraFields">
          <h3><?php echo JText::_('Additional Info'); ?></h3>
          <ul>
            <?php foreach ($this->item->extra_fields as $key=>$extraField):?>
           
<!-- insert code to test for registared and wether or not to skip here Replace HideField2 with name of your extra field-->
 <?php $user =& JFactory::getUser(); ?>
 <?php if($extraField->name == "HideField2" && $user->get('guest') ==1) { ?>
<!-- prove it works - delete the text below after testing -->
            field omitted
           
<?php  } else { ?>
            <li class="<?php echo ($key%2) ? "odd" : "even"; ?> type<?php echo ucfirst($extraField->type); ?> group<?php echo $extraField->group; ?>">
                <span class="k2ExtraFieldsLabel"><?php echo $extraField->name; ?>:</span>
                <span class="k2ExtraFieldsValue"><?php echo $extraField->value; ?></span>
            </li>

           
<?php } ?>
            <?php endforeach; ?>