You can control your party privacy from the Party Hub settings on your Fortnite application.
You can decide if your party is open for your friends and their friends to join, or you can lock the party so no one can join without being invited.
To lock your party, tap the lock icon on the main Party Hub screen. You can also unlock your party by tapping on the same lock icon.
You can also set Party Hub to automatically lock your party when you are playing alone by navigating to the Options menu on the top left of your screen.
Once there, you can toggle Private Mode on.