This bug seemed to be fixed when you did the final part of flame and frost, but this latest update seems to have re-introduced to the game,
Parties are again broken, you cannot enter dungeons together, as it breaks the instance, you can see each other in party chat, but not the UI window, this makes it near to impossible to do stuff as a team.
This bug is game breaking, and really does need a proper response from Anet do acknowledge thakittens being looked into.