As most of us know, the game died and the servers are down now, even though, a bunch of people like this game. If you still want to play it, get the standalone version of the game and download the maps pack file (links below), go to Multiplayer menu and select LAN, host or join a server there (I'm 90% sure portforwarding is not required in order to other players join your own server. Also, the other players must have the map which is being hosted, so you must download it to join a game). I have also made a Google Hangouts group so the players won't get lost at all (or just stick with one of the discord servers) since the game's chat is down aswell. Good luck!
Maps pack link (to install a map, simply drop them in the game's main directory) > Minimal: Click here | Click here (on this webpage, you can simply copy & paste the link from one of the maps and add their URL in the game) | Full: Click here | Click here (There is no need to download all of them, you can install the maps by their URL, like as I said before. Click on one of the maps, you will be redirected to the download page, right click the green download box in the page and copy the address, paste the URL in CSP)