This code snippet will redirect user from wp-signup.php on multisite to a custom page URL. Don't forget to replace Your_URL_GOES_HERE with actual target page URL.

redirect_from_signup_to_custom_page.php

<?php 
function sjaved_redirect_user_from_wp_signup(){
	global $pagenow;

	if( $pagenow == 'wp-signup.php'){
		wp_redirect( 'Your_URL_GOES_HERE' ); 
		exit;
	}
}

add_action('init', 'sjaved_redirect_user_from_wp_signup');