You should perform  the following steps only in case if you will use Call For price functionality for Grouped Products.

  1. Turn on template hints. How to do it, read here

  2. Open group product and define template path (default path is app/design/frontend/base/default/template/catalog/product/view/type/grouped.phtml)

  3. Open this template and find line with 'out of stock'. You should replace it like below:


Before:
<p class="availability out-of-stock"><span><?php echo $this->__('Out of stock') ?></span></p>

 

After:
<?php if($_item->getIsInStock()): ?>
<p class="availability out-of-stock"><span></span></p>
<?php else: ?>
<p class="availability out-of-stock"><span><?php echo $this->__('Out of stock') ?></span></p>
<?php endif; ?>

 
 4.  Save and clear cache.