Having just posted about Garcia's October 1966 sit-in with Big Brother and the Holding Company, I'll follow on with what is even more interesting-sounding (though it apparently didn't come off that well).
Mojo Navigator R&R News no. 9 (October 17, 1966) gives a brief account of the evening, which apparently included a configuration ("One Night Only!", I assume) called The New Peanut Butter Sandwich, composed of the following (instruments are mostly presumed):
Ron "Pigpen" McKernan (GD): ?organ?
Jerry Garcia (GD): guitar;
David Getz (BBHC): drums;
David Freiberg (QMS): bass;
?? (Sir Douglas Quintet): rhythm guitar;
Gary Duncan (QMS): guitar;
John Cipollina (QMS): guitar;
?? ("negro blues singer"): ??.
The only other detail is that "They did a very long and boring pseudo-Butterfield blues instrumental." Not clear what the blues singer was doing sitting in on an instrumental, but anyway ... sounds interesting!