Councils were very rare, as the System Lords spent most of their time combating each other. When faced with external and mutual internal threats, they would meet with each other in a form of parlay.
The Council had the authority to grant the rank of System Lord to a Goa'uld, and also banish a System Lord from their number (as was the case with Anubis and Sokar). A System Lord or powerful Goa'uld could also send a delegate to the Council and vote on their behalf. The council also had the authority to negotiate with an enemy of the Goa'uld Empire, such as they did with the Asgard over the Protected Planets Treaty, though the Council rarely pursued the negotiation option.
A second council was held when Ba'al offered the remaining System Lords the right to govern their territories in exchange for acknowledging him as Supreme Commander. This council was attacked by the Replicators led by Replicator Carter and all in attendance were killed. (SG1: "Fair Game", "Summit", "Last Stand", "New Order, Part 1", "New Order, Part 2", "Reckoning, Part 1")