<?php
/**
 * Disable Buddypress Registration
 */
function yzc_disable_bp_registration() {
  remove_action( 'bp_init',    'bp_core_wpsignup_redirect' );
  remove_action( 'bp_screens', 'bp_core_screen_signup' );
}

add_action( 'bp_loaded', 'yzc_disable_bp_registration' );

function yzc_redirect_bp_signup_page( $page ){
    return bp_get_root_domain() . '/wp-login.php?action=register';
}

add_filter( 'bp_get_signup_page', 'yzc_redirect_bp_signup_page' );