How would I allow players to join a place of a game without it creating a new, empty one?

I've worked on a story game before and when I'd create a new place for the players to teleport to from the lobby, it would teleport them to that place but empty, basically a new server. How would I change this? I want players to teleport from the main place/game to a sub place but without it creating a new server (so for example if there were 3 players in a sub place already the joining player would join them)