title: Apple Sign In - How to let users to invite others?
body: <p>I am developing a Swift based iOS app, which requires Apple Sign In. At some point, users may prefer to share some app-specific information with their friends or families. So they need to invite others via the app.</p>
  <p>Due to regulations of Apple, I may not reach the user email addresses if they don't share this information. The app uses Firebase on the backend, however what I can get is <em><strong>user name</strong></em>, <em><strong>privaterelay mail address</strong></em>, and <em><strong>userID</strong></em>.</p>
  <p>What I want is when a user enters an email address of her friend, I can match the data with other's. I may alternatively use some system generated code, but this will also require some additional apps such as Messages or Mail.</p>
  <p>Under this circumstances, do you have any clue, how I let users to invite others, without a painful method?</p>
  <p>Thank you in advance.</p>