The phrase that is used is "grass roots activists."
It is important to understand, however, that the above model is overly simplified. The grassroots and the establishment can often be the same. People can function simultaneously in either realm, and many often do. In fact, a large proportion of grassroots activists are current office holders or party leaders who are deeply engaged, alongside common citizens, in what could only be described as grassroots activism -- involving them in cooperative, advisory, or leadership roles with people of the lower echelons.
BS, grassroots activists who double as party insiders are in reality party insiders pretending to be grassroots activists.