I'm assuming this is a bug?
I know for a fact that you can travel to a player in any public dungeon in the base game, as that's what my wife always does when we were doing mage guild dailies.
And I also know it works in Vvardenfell and Northern Elsweyr.
But I've tried on NA and EU servers, and you can't travel to a group member who is in the public dungeons in Blackwood.
It just says no valid target found.