Wondering why you don’t see other players during free-roam and no one seems to join you on your missions? Well it’s probably because by default The Division turns your in-game privacy to private every time you log out and in.
You can change this setting by going into the game menu, then Group Management, Privacy on the bottom and then finally Open Group. This is certainly neat for people who play solo or when playing without friends. This will however not populate the overworld with hundreds of others players but will only fill out your party to its 4 player limit.
So in short, enabling Open Group will most of the time keep your group full by having matchmaking always on.
The Division by design is much more fun playing with other players so you should definitely have this enabled.