User registration

Im hoping someone can help. Im trying to add information to a seperate sql table when a user registers or when a user is added via the admin interface.

so far I have found this hook.

function plugin_post($login,$email,$errors){
global $wpdb;
$sql2 = "select ID from wp_users WHERE user_email='$email';";

$userID = $wpdb->query($sql2);
echo $userID;

}
this returns a numm user id as the information gather before insertion into wp_users.
I need to obtain the user_id post registration after the user have been added to the DB and need a way to call this directly after a user registers or a user is added via the admin page add user.

Thanks again.