you can distribute the application for free through the App Store, where you can only log in if you use a service paid by extern
I would say yes, if I understood your question correctly. You just need to get users to log into your iphone app and check that they are members of this external service you are talking about.
Yes. You will want to set up a test account so that the Apple App Review team can sign in to your app and test it upon submission for approval.