Abstract
When do agents in loose, unstructured groups have the joint ability that a collective obligation implies? Two agents a and b have joint ability to do x if a has individual ability to do xa and b has individual ability to do xb , where xa and xb produce x, both actions are compossible, and a and b are in principle capable of intentionally combining them. Importantly, joint ability is not the same as the ability to perform a joint action in the strict sense. Joint ability is scalar, and a plurality’s joint ability may be insufficient for grounding a specific collective obligation where the likelihood that the agents jointly produce x is very low.