have possibly found a issue with save file size and multiplayer. the larger the file the more likely it is that people won't be able to join a multiplayer game. aka my file size is 11mb and my friends is 9mb and we are able to join his game (including sending save back and forth to test for NAT related issues).
i know you guys are working on it. i just wanted to put out my observations on this. hope it helps narrow down the problem. thanks for great game