No component - Registration

bp-custom.php

<?php
/**
 * BuddyPress Custom
 *
 * Customize BuddyPress to only manage sign ups.
 */

// Exit if accessed directly
defined( 'ABSPATH' ) || exit;

// Do not create a user on registration
define( 'BP_SIGNUPS_SKIP_USER_CREATION', true );

/**
 * Only keep core members component
 */
function imath_restrict_default_component( $default = array() ) {
	return array_intersect_key( array(
		'members'       => 1,
	), $default );
}
add_filter( 'bp_new_install_default_components', 'imath_restrict_default_component', 10, 1 );

/**
 * Disable the members component if not registration
 */
function imath_disable_members_directory( $bool, $component = '' ) {
	if ( 'register' != $component && 'activate' !=  $component ) {
		return false;
	}

	return $bool;
}
add_filter( 'bp_is_current_component', 'imath_disable_members_directory', 10, 2 );

/**
 * Exclude members page from wp_list_pages
 */
function imath_exclude_members_directory_page( $pages = array() ) {
	$pages[] = buddypress()->pages->members->id;
	return $pages;
}
add_filter( 'bp_core_exclude_pages', 'imath_exclude_members_directory_page', 10, 1 );

/**
 * Do 404 if trying to reach the members page
 */
function imath_disable_buddypress_front_end() {
	if ( is_buddypress() && ! bp_is_register_page() && ! bp_is_activation_page() ) {
		bp_do_404();
		return;
	}
}
add_action( 'bp_actions', 'imath_disable_buddypress_front_end', 1 );

/**
 * Remove Submit metabox in wp-admin/extended profile
 */
function imath_disable_members_admin_major_metabox() {
	remove_meta_box( 'submitdiv', get_current_screen(), 'side' );
}
add_action( 'bp_members_admin_user_metaboxes', 'imath_disable_members_admin_major_metabox' );