The music at Woodstock wasn't limited to the hours that bands and performers were on the stage. The party went on every hour of the day, all around the site. This was a time when a lot of music fans also played music themselves and brought their instruments along with them. If you were any good, you might find you could draw a crowd yourself!
This clarinet and drum combination certainly seem to have the people around them up and dancing, and it looks like there may be a few more instrumentalists just out of shot. Guitars were played around campfires everywhere you moved. Who knows what stars of the future were entertaining crowds of five or ten with songs that they only wrote an hour beforehand? People went to Woodstock to get lost in the music, and the music duly obliged.