We have a multi-platform App that we're building, where people largely sign up on our website through landing pages, or over the phone. We're required to have In-App-Purchases, so we're adding that. We're struggling though on providing access to the rest of the multi-platform experience if we don't have an email address. For example, if they sign up via IOS for a product with In-App-Purchases, then want to sign in to our user portal - Any tips on what fields I can use, or has anyone here accomplished this and have some pointers?