Jonas's society is filled with rules about anything and everything. They even have rules about when a child has to give up his or her comfort object. The text isn't exceptionally clear about who established the rules. The Giver tells Jonas that many of the rules were established long before his time. He doesn't say who the original rule makers were.
The society is governed by the Elders, and they presumably make new rules, modify existing rules, and even get rid of certain rules. I would like to point out that chapter two does have a reference that some rules might be changed by a committee, and the committee isn't necessarily made up of Elders. Changing a rule isn't easy, and it apparently takes committees a long time to come to any consensus about a change:
There was talk about changing the rule and giving the bicycles at an earlier age. A committee was studying the idea. When something went to a committee for study, the people always joked about it. They said that the committee members would become Elders by the time the rule change was made.
That quote seems to suggest that the Elders have given committees composed of non-Elders the power and ability to establish or modify certain rules.