Agora is a nomic, a game in which the rules can be changed as part of the gameplay itself. The concept of Nomic, and an initial ruleset, was invented by Peter Suber, and published in an appendix to his book The Paradox of Self-Amendment in 1982; Agora started with a very similar ruleset (adapted for play via email) on 30 June 1993, and since then has become one of the world's longest-lasting nomics.
Over its history of over 22 years, Agora has seen over 7700 proposals, and over 1300 rules enacted; but rules are amended frequently and repealed at much the same rate they are enacted, meaning that the ruleset is often fresh and interesting for new and experienced players alike, rather than continuously accreting rules until the game becomes unplayable. Although the game has been won on over 60 occasions, it can continue on after a win, giving everyone a chance to succeed (not that winning is easy; most months, nobody accomplishes it). The course of the game is charted by its over 3400 judicial cases, via which disputes are resolved.
Agora is a relatively serious nomic; many of its players see it as an experiment in philosophy, political science, and group dynamics, rather than just a game; sometimes it acts more like a country. It tends to be relatively conservative as nomics go, in that major rules changes often take a while to happen and are commonly debated in advance. Unlike some other nomics, much of Agora's gameplay is concerned with the proposals system and the rules themselves, with many wins having been via exploiting loopholes in even long-standing rules, just as other players prefer to win in the "intended" fashion. However, it is not uncommon for substantial subgames to be proposed, played for a while, and then repealed when players bore with them.
On the other hand, Agorans tend not to be above silliness and having fun; some players will do things just to see what will happen, or to create unusual situations merely to observe the resulting chaos. Agora occasionally experiments with mechanics designed only to shake things up a bit (such as randomized rule changes, or offices whose only job is to try to repeal rules); and once went to war over an insult that Agorans were "a generally boring lot".
As of August 2015, there are three mailing lists (see following lists for details) most players subscribe to:
Although many players prefer to watch for a while rather than playing (with Agora having attracted quite a sizeable list of "Watchers", who sometimes join in discussion about the game despite not playing), it's entirely possible to jump right in and start playing. Before joining, you should probably read the rules; the most recent rulesets can normally be found in the agora-official archives, or online. Probably, you can join by simply sending a message to agora-business stating that you register, although as always in nomic, the rules are subject to change.