After 3 weeks from it’s initial announcement of an invite-only system to play the game, Epic Games just announced on Twitter that everyone can now play the game without an invite!
This might be pushed to catch up it’s rather famous counterpart, Player Unknown Battlegrounds (PUBG) which is available in both iOS and Android devices. While Fortnite, being an iOS exclusive at the moment, was also an invite-only game hence the fewer players in it.
You can download Fortnite on your iOS device in the App Store for free.