I found this thread by chance but what memories it brings back. MSG Folk Nights were a must-see for my drinking friends and me in the 1960s. After a few beers the harmonies sung along with the groups were incredible. Like "Whiskey in Jar-O", "Hand me the punch ladle, I'll fathom the bowl...", and "Old Pendle, Old Pendle, Thou Standest Alone, Twixt Burnley and Clitheroe, Whalley and Colne...". How I recall these lyrics so clearly 40-odd years later I can't imagine.
We delighted in singing along to regulars like The Pennine Folk and The Taverners and I think The Dubliners showed up one evening, as did Martin Carthy.
Someone earlier in this thread also mentioned The Grehan Sisters - we didn't see them at MSG but they were a supporting act to Simon & Garfunkel in their 1967 show at The Feee Trade Hall. The sisters were memorable for the most incredible performance of playing the spoons!