Final code for myCRED Tutorial - http://mycred.me/tutorial/buddypress-sort-members-by-current-balance/

sort-members-by-balance-tutorial

/**
 * Add Sort Members Option
 * @since 0.1
 * @version 1.0
 */
add_action( 'bp_members_directory_order_options', 'mycred_pro_add_sorting_options' );
function mycred_pro_add_sorting_options() {

?>
<option value="points-asc">Points Balance (Ascending)</option>
<option value="points-desc">Points Balance (Descending)</option>
<?php

}

/**
 * Adjust BP User Query
 * @since 0.1
 * @version 1.0
 */
add_action( 'bp_pre_user_query', 'mycred_pro_pre_user_query' );
function mycred_pro_pre_user_query( $BP_User_Query ) {

    // Only run this if one of our custom options is selected
    if ( in_array( $BP_User_Query->query_vars['type'], array( 'points-asc', 'points-desc' ) ) ) {

        global $wpdb;

        // Adjust SELECT
        $BP_User_Query->uid_clauses['select'] = "
        SELECT DISTINCT u.{$BP_User_Query->uid_name} as id
        FROM {$wpdb->users} u
        INNER JOIN {$wpdb->usermeta} um
        ON ( u.{$BP_User_Query->uid_name} = um.user_id )";

        // Adjust WHERE
        $BP_User_Query->uid_clauses['where'] = "WHERE um.meta_key = 'mycred_default'";

        // Adjust ORDER BY
        $BP_User_Query->uid_clauses['orderby'] = "ORDER BY um.meta_value+0";

        // Adjust ORDER
        $BP_User_Query->uid_clauses['order'] = ( $BP_User_Query->query_vars['type'] == 'points-asc' ) ? 'ASC' : 'DESC';

    }
}