If you want to share your LEGO Fortnite world with friends so they can play while you are offline, you can make them Key Holders of your world.
Follow these steps to do so:
- Press M to open the Menu.
- Go to the Players tab.
- Select the player(s) you want to share the key with and hold down the button prompt.
After that, your friends will be able to select your world to continue progressing while you're offline.