Organization
The structure of Terra is highly decentralised. While there might appear to be a sort of hierachy, this is mainly an instrument of coordination. People in any given area are simply more aware of the state and workings of that area.
Groups
It is quite possible to simply join the work as a single individual. In many areas, that will be the only option at first. But anyone is encouraged to assemble small groups in which to discuss the special needs and possibilities of their immediate surroundings. Not all in the group need be in contact with the rest of the organization, just as long as someone keeps a finger on the pulse, and in return inform the rest of the groups own results or problems.
Embassies
These are somewhat more "official". While a group just goes about its doings and keeps in contact from time to time, an "embassy" is continously open for anyone to contact (even if response might take a little while). Current embassies are merely to be websites keyed for the country, state or region in question, with an attached E-mail address. Both site and E-mail are meant for non-personal material, though, to avoid it mixing into peoples personal affairs and vice versa.
An embassy will, first and foremost, be presenting the Terran Alliance to the people of the given region (possibly including other regions lacking one of their own). But it is also a coordinator, keeping rough tabs on the doings of that regions groups. This is to ease interaction between groups, both in the given region and, through the help of other embassies, in other regions.
Terra Main
This site, and its crew. This part of the organization is meant to be a formal introduction to Terra, giving visitors an idea of the concept on which it is built. It is also a "default" embassy for people unable to find one suiting their region. Finally, it gives a rough idea of the status of work being done. It is not very personalized, though, which is why embassies are needed.
Like embassies, Terra Main is used for coordination. It is not meant to do continuous group coordination, but short term work affecting or drawing from many regions will be put here, since it is easier. As well, any event affecting Terra in general will be mentioned here. But the real work is still done in the embassies and individual groups and persons.