After some thought, I proposed the following:
You are all exiled from your own people due to some crime or infraction, and have been living rough among the humans and elves (who look down on you) for some time. You've all had enough and are now eager to make your mark in the Ravenlands.
It's not super-original, but it was enough for them.
My intention is to agree how they got together during chargen, so we can start en media res out in the Ravenlands. No meeting by chance in a tavern or in a caravan for me, thanks very much!