If you check the link Groups at the top you will find listings of what groups there are to choose from, and which groups you've already applied to and have been granted access to. This in order to have an incling where people are from and so on.
If a country is missing, it's probably because an official liaison haven't been appointed in that country, and haven't registered to this here site yet. If that's the case, feel free to ask regionally if the process of appointing one has been started, or suggest starting the process in case it haven't.
In order to join a group, choose it and press the "Join Group"-button. The group-moderator will acceptyour application. Please do not abuse this - and request membership in more than the group you actually belong to.