I was running Fractals of the Mists with a pickup group recently, and part of the way through the second fractal, one of the party members was somehow ejected from the dungeon. His portrait changed to a black silhouette and in party chat he said that he was outside the entrance and couldn't find a way to get back in. Even after we completed the second fractal, the party member said that he couldn't find a way to rejoin the group.
Is it possible for a party member to rejoin an instance of a dungeon that's already underway? Is there any difference between Fractals of the Mists and other dungeons in this regard?