Here is the answer of Herman Bavinck, as found in Reformed Dogmatics, Vol. 4, p. 431:
All these meetings reported in the New Testament were assemblies of the local church, attended only in Acts 15 by representatives from other places. This custom [of congregational involvement] was also followed later, as early as the second century. In important matters such as the appointment or deposition of a bishop, excommunication, the absolution of mortal sins, and so forth, not only did the presbyterium offer leadership, but the congregation also gave its consent. Cyprian still writes that from the beginning of his episcopate he did nothing without the advice of his presbyterium [or "body of elders," ibid, p. 430] and the consent of the congregation.83 Present at the synods of the second and third century, therefore, are not only the bishops but also the presbyters, deacons, and ordinary church members. Even the Council of Nicea was attended, aside from bishops, also by presbyters, deacons, and members who took part in the debates. And the delegates who were invited to congregational meetings of neighboring churches in that period were absolutely not only bishops but also included presbyters, deacons, or other members of the church. But the result of the development of the hierarchical idea was the consent of the congregation was increasingly less frequently requested, the presbyters and deacons were detached from the congregation and changed into counselors and helpers of the bishop, and the synods were gradually held only by bishops.