Disable BuddyPress' registration and use WP's instead. Paste this in /wp-content/plugins/bp-custom.php.

gistfile1.php

/**
 * Disables BuddyPress' registration process and fallsback to WordPress' one.
 */
function my_disable_bp_registration() {
  remove_action( 'bp_init',    'bp_core_wpsignup_redirect' );
  remove_action( 'bp_screens', 'bp_core_screen_signup' );
}
add_action( 'bp_loaded', 'my_disable_bp_registration' );

I suggest to add

  add_filter( 'bp_get_signup_page', "firmasite_redirect_bp_signup_page");
    function firmasite_redirect_bp_signup_page($page ){
        return bp_get_root_domain() . '/wp-signup.php'; 
    }

@unsalkorkmaz
the way you suggest doesn't work for me.
when I try with gistfile1.php, it works.
I don't know why your way doesn't work for me.

This method no longer appears to work, at least on multisite?

instead of using return bp_get_root_domain() . '/wp-signup.php';

use return bp_get_root_domain() . '/wp-login.php?action=register';

Even with the update that @fabianski7 added, this code has no effect on the BuddyPress registration page for me. Any other suggestions or updates?