Redirect to the registration page of the main domain
Follow below steps to solve your issue.


Install rules module in subsite.
Create a rule with condition as page being viewed is user/register
Add an action redirect to URL with url as main site user/register path


Other way to do is using custom module in sub site

Use hook_init in custom module

   &lt;?php
      function yourmodule_init() {
         if (arg(0) == "user" &amp;&amp; arg(1) == "register") {
           drupal_goto("http://www.abc.com/user/register");
         }
      }
   ?&gt;