https://www.gamespot.com/articles/fortnite-mobile-friend-invites-now-available/1100-6457474/

It seems that Fortnite's mobile test is expanding quickly. With the first invites having already gone out on iOS, Epic Games is now allowing select participants to begin bringing their friends into the game--so if you know someone who's in, now is the time to start bugging and/or bribing them.