When a team member dies and leaves, they can use /back to return to the dungeon.

Hey there. The /back is a command from another plugin, not Mythic Dungeons. If you want to prevent people from teleporting into dungeons, you can change the following setting in your dungeon's config:

Rules:
  PreventTeleportIn: true

With that set to "true", players won't be able to teleport into that dungeon unless they have the bypass permission dungeons.bypassjoin