This is how you add addthis to your group view.
Go to modules/boonex/groups/classes/BxGroupsPageView.php
and this code or the code of your preference at the last line below ?>
<!-- AddThis Button BEGIN -->
<div class="addthis_toolbox addthis_default_style ">
<a class="addthis_button_facebook_like" fb:like:layout="button_count"></a>
<a class="addthis_button_tweet"></a>
<a class="addthis_counter addthis_pill_style"></a>
</div>
<script type="text/javascript" src="http://s7.addthis.com/js/250/addthis_widget.js#pubid=xa-4d9baa2d2505efa7"></script>
<!-- AddThis Button END -->