custom filters for user list view

I have a set group of about 1200 users in my network (only 450 have logged in so far) and I would like to customize the user list view based on a custom-filter I created that allows the user to choose the building they work in. If I could add an option below, that would help people find their friends much more easily.

I found this switch in com_easysocial controllers, which appears to set the $option

switch( $filter )
{
case 'online':
$options[ 'login' ] = true;

break;

case 'photos':
$options[ 'picture' ] = true;
break;

default:
break;
}

I also found in theme->wireframe->user the file default.list.php which accesses <?php foreach( $users as $user ){ ?>

What I would like to know: where do I find the function that pulls users from the database according to the preset options?

something like "sql get userID where userid.customfield equals "harrison"

Any advice appreciated!