• If you already went to Last Light Inn and started the quest with the Harpers, you can no longer join the patrol. You're forced to attack it.
  • If you never went to Last Light Inn and just follow Kar'niss's patrol, the Harpers will show up down the road to ambush you (just like you would normally ambush the patrol yourself with the Harpers) and you'd be presented to either defend the patrol or join the Harpers.
  • There is also a way to join the patrol without killing any Harpers by not summoning Kar'niss at all and just going down the road from them towards Moonrise, at which point Kar'niss's patrol will spawn on top of you and you'll be able to join them without the Harpers showing up.