[kh99]

There is a hook location paidsub_build, which is in inculdes/class_paid_subscription.php around line 453. But it's after the usergroup has been changed, and it isn't only called once when the subscription is created. It can be called when it's renewed, or when another subscription is deleted (I suppose that's to make sure that the user ends up in the correct usergroup if they have more than one subscription, and one is deleted). In any case, there might be something you can check to know if you want to do whatever it is you want to do.