Create a log of create/delete users in Drupal 7
You could actually log the details about user creation at hook_user_insert(), and deletion using hook_user_delete().

User creation: Acts on new user creation

function MODULE_user_insert(&amp;$edit, $account, $category) {
  watchdog('user', 'User created with ID: %userid', array('%userid' =&gt; $account-&gt;uid]), WATCHDOG_NOTICE, $link = NULL);
}


User deletion: Acts on account deletion

function MODULE_user_delete($account) {
  watchdog('user', 'User having ID: %userid deleted', array('%userid' =&gt; $account-&gt;uid]), WATCHDOG_NOTICE, $link = NULL);
}