Sorry for the long reply. To be honest, I don't know what's appropriate to write. Usually I do something like "私は[your name]です。[community leader]の大ファンだから,コミューをはいたい。よろしくお願いします。" (I am _____. I want to join because I'm a big fan of ______. Nice to meet you.

To be honest I have no idea if that's appropriate or effective, sorry :(