What mission is selected in co-op?

I've scoured the forums for an answer to this and haven't found one so here goes...

Let's say I have mission A in my quest log. I invite someone to party up who has mission B in their quest log. What mission gets priority?

1) Mission A because I formed the party?
2) A or B, depending on what we decide to do?
3) Whatever mission I have highlighted in my quest log?
4) Whatever mission we are in the right area for?

Does my party member get the benefit of completing quest A, even though it's not in his log?

And let's say when we're out in the field, I talk to an NPC and get given mission C.
1) Do all the team members automatically get the same quest?
2) Do they have to individually talk to the NPC?
3) Or am I just supposed to tell my party members what the quest is about?

Thanks!

I think this part needs reworking in the game.

You don't see other people's quest arrows, only your own. so the best thing to do is talk to your party and see what quests they have. If you have the same quest, then just follow that arrow. if you are helping someone out with a quest you don't have, you won't see their arrow so you will just have to follow them.

NPCs in the field, each member of the party will have to talk to that NPC to get the quest so that you all get the end of quest XP. otherwise it is the same as above.

I found your post a bit confusing at first until I realised you weren't talking about missions but were talking about quests. They are 2 different things. I'll try to answer some of your questions about QUESTS.
Neither have priority. Where you go and what you do is decided by the group. You can play the dictator and tell the other members of the group what you're going to do, but that will likely result in members leaving because it's not what they want to do. My suggestion would be to be clear before they jion that the group is being formed to do quest A and not quest B.

No they don't you only get credit for quests that you've got not someone elses.

All party members must talk to the NPC to receive the quest.

I hop this helps.

I had a quest experience where a quest item (poison stinger) was assigned to me, so I was able to complete the quest, but my friend still had it open. If there's a quest item drop after you kill something, there should be as many items as people in the party, so everyone can complete the quest.