Okay, so have been playing since beta, and grouping has always been at best an imperfect process in ESO. Long ago I came to peace that the main quest, Mages Guild Quests, and Fighters Guild quests are solo-only play ... don't like it, but accepted it as something ESO simply has no plans on altering.
But let's look at grouping otherwise: If you're grouped with another player, that player can be phased out of existence if your own character has already completed a quest. You'll see their indicator running about, but that's it.
So where does that leave a higher-level character helping out a lower-level character? Well, it doesn't ... unless you avoid all phased quest areas.
Then there are the groups space-barring through dialog from quest-giving NPCs, leaving behind those who want to hear what the NPCs have to say.
SWTOR got a lot wrong, but one thing they got right was grouping. You could be a Level 50 character and group with a Level 2 character, sharing all his or her missions. And if you space-barred though content, you still had to wait until the other group members were done before racing off to do battle.
These should be easy fixes for the developers of ESO.
Crotchety Old Man Guild
"Hey you, get off my lawn!"