From my understanding signing up within the App isn't an issue as well. The question is, can the user use the App at all without paying?
I have a mobile App that has our own private "cloud" storage. The user can use the App to their hearts desire. But, if they want our "cloud" they must subscribe via our Web portal.
But, if they don't care about that - they can use the App indefinitely. I don't think there is a steadfast rule to it, all Apple seems to care about is if the App can be used (functional). Where, Netflix as an example fails at that end, because you can't use the App without having a paid Netflix account.