[imuhammad]

// code for functions.php

function xfield_member_filter( $field_name, $field_value = '' ) {
  
  if ( empty( $field_name ) )
    return '';
  
  global $wpdb;
  
  $field_id = xprofile_get_field_id_from_name( $field_name ); 

  if ( !empty( $field_id ) ) 
    $query = "SELECT user_id, value FROM " . $wpdb->prefix . "bp_xprofile_data WHERE field_id = " . $field_id . " ORDER BY value" ;
  else
   return '';
  // var_dump($query); die;
  if ( $field_value != '' ) 
    $query .= " AND value LIKE '%" . $field_value . "%'";
      /* 
      LIKE is slow. If you're sure the value has not been serialized, you can do this:
      $query .= " AND value = '" . $field_value . "'";
      */
  
  $custom_ids = $wpdb->get_col( $query );
  
  if ( !empty( $custom_ids ) ) {
    // convert the array to a csv string
    $custom_ids_str = 'include=' . implode(",", $custom_ids);
    return $custom_ids_str;
  }
  else
   return '';
   
}

function alphabetize_by_last_name( $bp_user_query ) {
  ;
  if ( 'alphabetical' == $bp_user_query->query_vars['type'] ) {
    $bp_user_query->uid_clauses['orderby'] = "ORDER BY FIELD(u.ID," . $bp_user_query->query_vars['include'] . ")";  
    $bp_user_query->uid_clauses['order'] = "";  
    var_dump($bp_user_query);
  }
}
add_action ( 'bp_pre_user_query', 'alphabetize_by_last_name' );

// code for loop
<?php if ( bp_has_members( xfield_member_filter( 'Last Name' ).'&type=alphabetical' ) ) : ?>