S1x Private Match Guide:

1. You can go to here to get your IP Address and message anyone who wants to join it.
2. Start S1x and launch a private match.
3. Tell anyone who wants to join to open the console by using the tilde key while at the main menu.
4. Get them to enter the following command, replacing 'HostsIPAddress' with the IP you previously sent him: /connect HostsIPAddress:27016

Failed to join:

1. If you followed the above correctly and your friends are unable to join them you the host of the private match will need to port forward in order for them to connect to you.
2. Port Forward port 27016 UDP & TCP
3. You can find how to do this by Googling 'YourRouterName Port Forwarding'
4. Note: If port forwarding is too complex, just stick to playing public servers, or maybe join an empty server so you have the same experience as a private game.