An informal association of a few people sitting around a table does not have to have any rules at all. Each situation can be negotiated around as it occurs. Most groups that last longer than a few meetings do find it useful to lay down some rules, just so that everybody knows what the boundaries are. If the rules cause problems they can be changed at any time.
If one wants to go a stage beyond that, some of the rules can be made harder to change - some changes may have to be supported by a two-thirds majority, or have to be adopted after polling all the members. This means that people who join up can have some kind of certainty that the boundaries will stay where they are. This deliberately restricts a group’s flexibility. That's a constitution.
All incorporated not-for-profit organisations must by law have a constitution (also sometimes known as Rules, or Articles of Incorporation, or Articles of Association).
Click the link below to download the JAA Constitution PDF document.