I just submitted an app to the store which uses accounts to log users in to see billing info and such. I'm wondering if just emailing them the guest account info is enough, or what will happen when they begin reviewing the app and need to login.
There's an text area called 'Review Notes' in the metadata of an app in iTunes Connect where you can supply details about test-accounts and such.
If you already have a 'guest-account' available from the app and they can use the app (even if limited) that should be good enough for passing the Apple review.