How get phone position from other in real world?

Hi. Im making AR multiplayer game, and get some problem.
When 1st player joined game, he will spawn in (0,0,0) position. After, than 2nd player join game, i want spawn him in position, where player is in real world from 1st player.
I mean, if distance between players in real world is 1 metr, the 2nd player should spawn in (0,0,1)..
How can i detect distance between players in real world, when 2nd player want to join game(Prejoin process)?