I ended up winging up with a random CMDR in Eravate the other day myself, he was having trouble getting ganged up on in a salvage mission.
Problem is, nav-lock didn't work and when I tried to follow his low-wake instead it said "need scan". For a low-wake. Seriously. It wouldn't even let me drop on it and he got destroyed while I was trying to wrestle with getting into the instance.
So while my usefulness was limited to giving outfitting advice for next time and not to be worried about abandoning a mission if it's too tough, I still got to feel like I was helping. I think my next project is gonna be to make myself some sort of support ship and park it at Cleve Hub for newbie-helping duty.
Not sure if this applies to that mission type, but I think he has to share the mission using transaction panel and then you have to accept the mission as well.
It certainly applies to wing massacre missions,, and I could be wrong (I often am.)