I have a Pyromancer at level 58, I play alone and have very little HP. With that said, I was doing quests in Kvailas Fores when a Linker connected with him and another character, but I was not in his group, so also took the damage of the group, even if not with them. And it happened several times with different people.
My suggestion is that the Linker can only connect characters of the same group.
It is very unpleasant to lose HP for someone else and you do not know him.