One companion died in the first robber elimination (largely through friendly fire, grr), but my experience is that they usually refuse with the message that they won't join until <X> returns. I did later get a flat refusal from one adventurer, and it might be in the village the dead character came from.
I did eventually get 10 people to take on 5 robbers, so it worked out in the end (and I then proceeded to remove another known robber band of 4 when I had a posse at hand, plus a bonus Njerp). However, I think at least 4 former companions didn't want to join "then, but possibly in the future". Did I pay them too much so they didn't want more food?