Hi, I was wondering, how does the crossplay works between different platforms, do you need to add your friends so they can join you ? And is it possible to play with Steam, gog and epic games users at the same time ?

It looks like it was patched in and requires both users to use an epic account.
gog.com/forum/general/new_dying_light_update_enables_crossplay_between_gog_steam_and_epic/page1

I see, thanks