So yesterday I was quite happily questing on my own in the misty mountains and all of a sudden I had this person latch onto me from Gloin's camp and follow me around. It was all quite disconcerting. She/he didn't say anything to me. No "Hi, do you want to quest together?", just silence.
I thought they'd get bored after a while and go do something else, but nope. Still following me. So naturally, I thought maybe they needed help on a quest or something and for some reason couldn't ask me. I asked, politely, why they were following (I'm not opposed to have a quest buddy, I'd just prefer them to ask me first!), and got the answer "Because of open tapping".
I didn't exactly know what it meant, but as I understand it now, any person who contributes damage (no matter how much) gets the xp for killing a mob. (And loot aswell?)