I can highly recommend the Paint Creek Folklore Society in Rochester, Michigan (in the Detroit metropolitan area). (I was a member for years, until I moved to the UK.) Meetings are first Saturdays -- in the evening, in the charming Lawnridge Hall, which is a sort of house attached to a church -- a very cozy place to have a meeting.

PCFS is a bit unusual (at least in the US) in its emphasis on participation; in my experience, most US folk clubs/organisations mainly organise concerts rather than expecting members to sing and/or play -- perhaps this is why you haven't had many US clubs suggested on this thread so far. (You might also want to be forewarned that beer is not an integral part of the US folk scene as it is in the UK (and -- I assume -- Oz & NZ).)

Anyway, the evening starts off with a short (45 min or so) "workshop" -- usually on some aspect of folk music, dance or folklore, then there's a refreshment break (no beer, sorry!), followed by a song swap. The song swap has a "theme" and anyone is welcome to get up & sing a song or play a tune. There is a wide range of performing experience represented -- from beginners to pros, but everyone gets a warm reception. BTW, don't worry too much about the "theme" -- it's rather loosely adhered to; once when the theme was "Cooking", someone did a song that "I thought of while I was cooking..." ;-)

After the song swap, the "official" meeting ends and jamming commences; Lawnridge Hall has a bunch of little rooms which are perfect for small jam sessions. There are usually at least two: a big one downstairs (involving quite a few hammered dulcimers, as well as guitars, fiddles, etc) and a "singing jam" in an upstairs room.

Well worth getting to, if you're in the area.

Cheers,

YY