Might be worth noting here that formal "Folk Clubs" are nowadays much rarer (at least where I am) than pub singarounds / sessions / general music nights. These are generally much less formal than clubs, require less organisation and tend to be more inclusive of "non folkies".

My session has been going on for 12-13 years now. Mostly singing but with occasional instrumental outbursts. People in the bar, particularly the regulars (i.e. regular drinkers), frequently join in with both chorus and the song if they know it. Some of them (like Charlie, who has taken to bringing her flute) give us a song or a tune during the evening or borrow a guitar to do something with us.

A small group of us form the core, and we tend to practise together and perform at other venues (when we feel like it), but there is no membership, and there are no fees (though you can buy me a pint if you like).

:-)
Ian