The Routledge Handbook of Collective Intentionality

The concurrence criterion might be too strict. Some joint action is fluid, forming spontaneously and dissipating just as easily.

The idea is that one cannot “without fault” withdraw from (or substantially modify) shared activity unless every other participant concurs (Gilbert 1990, 2009: 174, 2006: 106–15).

Participants should, of course, display some sensitivity in their behavior to playing a role in shared activity. But to require that they understand it as such would rule out joint action for many who do, in fact, engage in it.