How to detect the closest players in an online and real-time game

Hello.

My problem is that I have to detect the closest users from a device, in order to implement the function of fighting other players in my game, currently in the manufacture of my game I am using Facebook services for Social and PlayFab to store the data of each user, but I also plan to use Photon for the part of the combat between characters and worlds, and I have no idea how to carry out this process of detection of other players, which I require to allow a player to choose a contricante and fight it

If what I'm looking for is at least an idea of ​​how to achieve my goal

Excuse me if my question is very simple or of little material, but this is my first project with online support and in real time.

Thank you.