Add a group sidebar filter

Hi guys,

I'm looking to add a "Paid" filter on the sidebar of the default group layout. See that theme should be:

<li class="filter-item<?php echo $filter == 'paid' && !$activeCategory ? ' active' : '';?>" data-es-groups-filters-type="paid">
<a href="/<?php echo FRoute::groups( array( 'filter' => 'paid' ) );?>" title="<?php echo JText::_( 'COM_EASYSOCIAL_PAGE_TITLE_GROUPS_FILTER_PAID' , true );?>" ><?php echo JText::_( 'COM_EASYSOCIAL_GROUPS_PAID' );?></a>
<span class="es-count-no pull-right" data-total-paid><?php echo $totalPaid;?></span>
</li>

i) Where is the method of this view to set totalPaid? view.html.php?
ii) What do I have to do for the group route (filter) to have it work?
iii) Any other thing I should know (corresponding method to adjust?!) in order to have a new group filter base on my paid group value to work? (beside language token)