<?php
function rogerthat_adminmods_only( $query, $sql_parts ) {
  global $wpdb;

  // Only do stuff if the user parameter was set
  if ( empty( $sql_parts['user'] ) )
    return $query;

  // Add a requirement that the matched user is an admin or a moderator of a group
  $sql_parts['user'] .= $wpdb->prepare( ' AND (m.is_admin = 1 || m.is_mod = 1 )' );

  // Rebuild and return the SQL
  return join( ' ', (array) $sql_parts );
}

// You will need to add_filter just before you want to make your special group query, and remove_filter just after, otherwise it'll affect all BP Groups loops.
// YMMV; this may also stomp over any other plugins that affect the Groups' SQL queries directly.
add_filter('bp_groups_get_paged_groups_sql', 'rogerthat_adminmods_only', 20, 2);
?>