When joining another player, it seems sometimes that they are in a previous version of a quest, where enemies may be spawning a normally friendly place or small mini-story elements, and when this happens you're unable to actually join them. You can find their location, marked by their normal arrow, however they aren't actually there
and you aren't able to help them...
i understand these parts are "meant" to be done in a single player scenario, however, people are able to join you if they have the quest as well, them just doing it alongside you.
in reality, it ends up working in a very backwards fashion, where sometimes you can run through a quest with the people around you basically completing it for you, then you can't go back to replay that quest line when a friend is on it alone, or otherwise needing help.
again, it's very confusing trying to link up with a friend, only to find out they're in an early part of a city quest that you completed long ago and you simply cannot see them, and kind of counter-intuitive when you think of it in the context of elder scrolls Online...
My suggestion: if I purposely join on a friend, i should be transported into their "quest status," giving me the same monsters (with reduced or removed loot, i don't care) and the same objectives, however with the text (completed) next to it.