John the Baptist is, so far, the only John we’ve encountered, so presumably these are the disciples of the Baptist. The Pharisees were populist conservatives who tried to keep the Sadducee Temple leadership in line with Hebrew traditions. The question is whether Jesus is representative of the separatists, like the Essenes, who felt themselves to be more purely Hebrew, in repudiation of the Pharisees and Sadducees. If both groups were fasting, one could presume that some sort of religious and/or calendrically indicated fast was occurring. This time, instead of merely thinking about their complaints or mentioning them to others, the Pharisees come directly to Jesus with their complaints.
Going from the identification of John the Baptist as John Hyrcanus II, the aged and widely respected Hasmonian prince and high priest, the identification of the “disciples of John” would specifically be the Sadducees.