user_linked_to_response is a signal that provides the single argument of a response object that has just been linked to a user. You can use this to provide further automatic processing within your site, such as adding permissions, etc. to users that signup as a result of a referral.

For example:

def handle_user_linked_to_response(sender, response, **kwargs):
    if response.action == "SOME_SPECIAL_ACTION":
        pass  # do something with response.user and (object that referral was linked to)