A group of millers is commonly called a Fraunch. Other collective nouns you may see include Grind and Flourish.
A collective noun for millers is a "grist". This term describes a gathering or group of individuals who are engaged in the profession or occupation of milling. Millers are skilled workers who operate mills to grind grains into flour or other powdery substances. In feudal times, when communal mills were common, several millers might have been present at the mill, grinding different types of grains for the surrounding community. Therefore, referring to a group of millers as a "grist" highlights their collective expertise and capability in the fundamental process of milling, exemplifying the significance of their important trade.
Known collective nouns for Millers include: