Bel and I tried to go into a fighters guild instance together, we both had the quest and when we went in, we could see our group icon but not each other, so i emailed ESO;
The ?;
A guild member and i tried to do this quest together, but when we both enter the portal we can not see each other.
Reply;
I'm sorry you can't see your fellow guild member in your instance! I personally am very into the multiplayer part of MMORPGs, so I can definitely understand your frustration when it comes to not being able to experience the game with your friend.
This can occur when you are both on different parts of a quest or when the quest or instance is solo only. I do believe this quest is solo, so you may be out of luck. Even though you cannot see each other, you may in fact be able to fight the same enemies, so I suggest trying to do the quest at the same time.
So it looks like Guild and Main story instances are solo only.