This feature is built into the game now! You can access it from EDIT GAME SETTINGS > ALLOW DROP-IN > STEALTH ONLY
Allow players to join your game while playing (if drop-in is enabled), but don't let them drop in during stealth until you press the safe key. They will only drop in if they've fully loaded and sync'd with other players, otherwise you'll have to press it again once they're ready.
Please don't forget to set the key! Don't forget to press it either, because people will leave.
