by-laws agreed upon

Several years ago, we were asked to help with the merger of two community volunteer groups, who wanted to join together in order to establish an agency that would have more influence and better financing than the two separate agencies of the past. Both were rural groups, which had traditionally and geographically been separated by a range of mountains.
We worked with the two groups to help them set goals, develop policy, and build an organizational structure. Meanwhile, the young executive director, who saw a successful merger as his primary responsibility, was especially careful about one thing. On either side of the mountain was a large town. Each of the former agencies had its own board, a separate staff, and long-standing regional ties. The executive director understood that the question of where the new joint agency's headquarters would be was an important question as well as a potentially explosive one.
In leading the planning meetings of the two boards, he carefully kept the groups focused on the long-range issues, and convinced them not to look at the headquarters question until the very end of the process. Once the organizational structure was set, by-laws agreed upon, and all the volunteers had actually merged, then the headquarters site was decided on -- logically and easily, as it turned out.

What does 'by-laws agrred upon' mean? I don't understand it's grammatical structure.