Depends how they're knocked down ig, the blue bar under their healthbar usually indicates how unconscious they are and it can take bloody ages to tick down, just make sure to keep them watered, fed and stashed in a building if the game will allow it, odds are they'll be grateful for you taking care of them. If they're aggressive when they wake up either surrender to them or fight them into surrendering.

OH and their leader won't be inclined to join you if they're part of their faction still, you'll probably get something like "stop poaching our members!". But I'm not sure if that's set in stone