Enter countries at random. If a country is a member of the club, it will appear in the blue column. If a country shares a border with one or more of the member countries, it will appear in one of the green columns. As soon as you think you know the secret rule, enter it to solve the whole quiz. (Several different wordings of the rule will work, so if it doesn't work first time, just try another wording.)
In order to create a playlist on Sporcle, you need to verify the email address you used during registration. Go to your Sporcle Settings to finish the process.