How can I get my iOS app approved and avoid the 30% Apple Store Fees?
Speaking from some experience -- Apple is pretty strict about this requirement, and will not look kindly on attempts to circumvent it.


  Do I just put a login screen on the app and then text below it that says "An account is required to use the app. Visit our website to register”


Apple will probably object to the language "visit our website to register", and may additionally reject your application because it is nonfunctional without an account.

Apple has been known to make some exceptions -- for example, Netflix requires a login and has no in-app signup. This is likely to be a special case, though; it's unlikely that you'll be able to have the same rules applied to your app.


  Alternatively, can I put an actual link there as long as it's not to a shopping cart page? Such as "An account is required to use the app. Visit www.example.com to register.”


No. Any link which can be followed to a signup page -- even if it's a circuitous path, e.g. a link to a terms-of-service page which links to a support page which links to your home page which links to a signup page -- may be used as grounds to reject your app.