From reading a bit and playing beta, I guess your world does not update if you do shit in another persons world. Meaning, lets say you group up with your buddy sam, and you enter sams world (he is group leader). You do a few of the control points, you also give some items to the contol point bosss to upgrade them, whatever. Then sam leaves, you go back to your own game world, and none of that shit carries over.
So I think what makes sense is to do missions and stongholds in group, but solo most of the other shit in your world.
At least thats my understanding of it.
There is also a reddit post about this. Person was asking about this, why his shit was not the same as in his buddies world.
Also the way levels work in this game, is that you are normalized to the highest person in your group. So lets say you are lv 2 and your buddy is lv10, you decide to do a mission with him, so you are leveled up to 10, and the mission is lv10 mission. I think the loot dropped is still to your level though, so you will be getting lv2-3 loot while he gets lv 10-11 loot, and so on.