realtime spectator mode on smartphones-based AR+board game

Dear Community,
we are building a smartphone-based AR game, built over a real game board. We'd have at the moment only one player in the game, but we'd like to offer other ones the capability to view what's going on in the virtual counterpart, in a sort of "spectator mode".
What is the simplest way to enable other smartphones to view in realtime (or semi-realtime) what is happening?

Thanks in advance.

Kind regards

Paolo