Elven Council of Shael i'Vor
The Elven Council of Shael i'Vor is a convenient term for an entity that doesn't quite lend itself to such definitions. While its primary purpose is to bring together the many different Elven communities of the forest, its true nature is more complex.
Shael i'Vor itself has no central governing body. Instead, it has, over the centuries, developed a system of bringing together the diverse groups of Elves to discuss and decide issues of mutual interest. Calling this arrangement a "council" may be overstating things a bit, and indeed the Elves have no name for it that translates easily, but it has worked for them for centuries, if not millennia.
The council can be called at the behest of any of the myriad Elven communities of Shael i'Vor, be they of Grey, High or Wood Elves. Meetings of the council take place in the City of the Trees, unless otherwise specified. The leader of the community calling the council sends messengers to some or all of the other communities, requesting them to gather at a certain time to discuss whatever issue is at hand. Typically, issues that call for a meeting are those that affect the entire forest, or at least some large portion of it, such as invading forces, plagues, large-scale calamities such as forest fires, or high crimes against the community, but some particularly chaotic leaders have been known to call meetings on a whim.
Participation in such meetings is not mandatory. Each community then decides whether or not to send a representative to this meeting, and then the council discusses the issue and comes to a decision (or at least attempts to do so). The key is that each community agrees, whether they send a representative to the meeting or not, to abide by the decision of the gathered council as it pertains to the issue outlined in the original meeting request. Of course, each community is also allowed to interpret the request as they see fit, so in practice such agreements are difficult to enforce (and, indeed, most Elves would be loath to enforce them anyway), and disputes are handled by exactly the same system.
Despite its apparent difficulties, the Elves have found this to be quite an agreeable system.