The player hosting the game has to open the session for other players. To do so he has to load the game, then open the game menu (Esc), Manage session -> Session settings -> Session type. Session type should be Friends only. Afterwards Epic-friends should be able to join the hosts game. Alternatively the host can invite players. To do so open the game menu (Esc), Invites -> Invite Players. The host can then invite players from his friends list.