Players who are interested in Fortnite: Battle Royale on iOS devices can sign up for the Invite Event (beta) right now. Epic will send out e-mail invites, with a download link to the App Store, soon. Players that get an invite will also get some codes to share with friends.
During the coming months, more players will be added to the test. Fortnite: Battle Royale on mobile requires iOS 11 on at least an iPhone 6S/SE, iPad Mini 4, iPad Pro, iPad Air 2 or iPad 2017 device. You can sign-up here. Registration for the Android version of the game is "coming later".
Fortnite Battle Royale will feature cross-play and shared progression across all platforms, including Xbox One.