Starting on Android Chrome 80 (to be released 2020-02-04T02:00:00Z) Discourse will be able to invite users from your phone contact list.
We use feature detection and graceful degradation, so the retro way of manually typing emails still works, but if we present the contact picker button on the UI, clicking on it will open your device contacts app and allow you to pick one contact.
We filter only contacts with emails registered, and limit the selection to only one contact at time so the workflow is the same.
If you want to test it right now, it is live on latest Discourse, and naturally here on Meta. But until Chrome 80 releases you need to use their Dev or Canary channel.