There are two methods to do this, one using the in-game friends list and the other using the console to connect directly.
Method 1 - (Friends List):
Note: Requires you to have each other added as friends on Steam and to have steam running.
- Start iw4x and select 'CREATE GAME' from the main menu and click 'START GAME'
- Tell your friends to press the F key on the main menu to bring up the in-game friends list
- Tell them to double click your name to join your game (If they hear a chicken noise, see "Failed to join")
Method 2 - (Connect Command):
- Go to WhatIsMyIpAddress.com and PM anyone who wants to join your IP Address
- Start iw4x and select 'CREATE GAME' from the main menu
- Click 'START GAME'
- Tell anyone who wants to join, open the console by using the tilde key while at the main menu
- Get them to enter the following command, replacing 'HostsIPAddress' with the IP you previously sent him:
Failed to join
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.
- Port Forward port 28960 UDP & TCP
- You can find how to do this by Googling 'YourRouterName Port Forwarding'
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.
This is the Tilde Key on your keyboard, however if have a keyboard from a different region please consult google images to learn what key to press on your given keyboard:
Handy website to lookup your IP Address (You want the IPv4 address): WhatIsMyIpAddress.com