First, start a game. With the game started, activate a server, taking note of the UDP port that you use. Also, unless you want random people wandering in, you should not list your game as a public server. If you've got a router on your home network, you need access your router controls and forward the chosen UDP port. This is a potential security risk, so make sure you make a note to yourself to disable that port forwarding when you're done playing. If you're not sure what the local IP address is for your computer, bring up a command window and run ipconfig. You'll need to know your computer's external IP address, which you can do by typing "what is my IP" in a Google search. Now give that IP address to your friends in some way, perhaps over your voice comm, or using an e-mail or messaging, so they can type it into the appropriate box on the manual IP connection screen inside the game.
Well, that does work, but it's a lot more effort than it should be.
While RWR feels like a simple game at first, there's a lot of complexity hiding underneath, with tons of different choices to be made from moment to moment. For example, the type of primary weapon you choose greatly affects the way you will approach the game. You can equip shotguns for short range, squad guns that have to be set up before you can fire, single-shot sniper rifles if you want to try to keep away from danger, and so on. Also, there are vehicles you can drive, some that are really just transportation, but also armored vehicles with gun turrets. Should you aim to drive around in a powerful tank-like vehicle, or should you and your squad hoof it? When you kill an enemy, his gear lays on the ground. You could pick it up and run it back to one of your supply trucks and sell it for more resource points, but that will take you away from the front. Is that a good idea?
Just remember that while you're pondering these possibilities, you're always just one bullet away from your demise. Actually, I should say that if you are wearing a protective vest, it's possible to go into a "downed" state, where you crawl around, presumably screaming out in pain, until somebody comes by with a medic kit to put you back on your feet again, or until an enemy soldier put you out of your misery. But make no mistake, in this game, you will die early and often. In that sense, it almost feels like a roguelike game, not because you have to start over, but because of that feeling of tight roping as far as you can before you fall. When you have stayed alive for a long time in RWR, you can start believing you've reached some new, higher plane of gaming--but then you are felled, often from a unexpected direction, and before you even know an enemy is nearby or that someone shooting at you.
All in all, this is a good game, and an interesting game, one that doesn't play like many others. But on the whole, I admire the game more than I enjoy playing it. I think the competition to setting a co-op games highlights much of I don't like about RWR. The game is complicated, which is fine, but the game hasn't been designed to guide the player through the complications. Many of the default controls don't make a lot of sense or feel arbitrary. And, although you can have a primary and secondary weapon, the only way to check which weapon you are currently using is to examine your tiny little soldier on the screen. And there's large aspects of how the game actually works and I certainly would never have understood unless I read through the wiki (which I highly encourage you to do also, if you try this.