The Division is only a few hours old and players have already found a way to grief each other. Redditors are reporting that in the game's first safe house is a doorway that a single player can stand in and block every other player from passing through. New players have to pass through the doorway after meeting their handler Faye Lau.
If you try to run at the blocking player you will phase through them after a couple of seconds but walking won't have the same effect.
I had a quick test and it works. All it takes is a single person to block every player that passes through the game's safe house.
(I'd advise against testing it yourself, unless you want a lot of pissed off players giving your a horrible stare.)