I have written of a group coupled in music-making as a single actor, a single physical system in which some signals pass internally within the nervous systems of individual participants while others pass externally between them. The external signals are transmitted as acoustic waves while the internal signals are transmitted as electro-chemical signals. Furthermore the individual’s movements, and hence their neural activity, must be synchronized at the level of 10s of milliseconds in order for the coupling to be effective.
That’s one aspect of coupling. There is, of course, another. When individuals enter into such a group, the give up many degrees of neural and neuro-muscular freedom otherwise available to them. We can think of this as an intentional characterization of the phenomenon that the previous paragraph characterizes physically.
My primary account of this coupling is given in chapters 2 and 3 of Beethoven's Anvil: Music in Mind and Culture (Basic Books 2001). You can download a prepublication draft of those chapters HERE.