Whenever new user registered, how to make them active based on some condition automatically
Using PHP on admin end is not recommended. If you are interested in custom module here are the pointers


Implement hook_user_presave : Invoked when a user is created or updated.
Make sure your code inside presave hook is conditional for new account created but not updated. You can use in_new property of account object

if ($account->is_new) {...}
Use Entity Field query to find a node with email id of current account being created
if you found a result then Set $edit['status'] as 1.