There are three popular ways to play online.
First you can use the Hamachi Client. Once you download and install Hamachi, you disable it in your firewall. Then you either create a network for someone to join or you join someone else's network. Once you are in the same network, one person hosts a room in crusade and the other right clicks the host's name in Hamachi and hits "Copy IPv4". Paste the IPv4 into "Join" in crusade, and you're golden.
Second way is through the Evolve Client. I don't know much about it, but it's about the same as Hamachi.
Third is through a Direct Port. You can set up your router to directly connect between computers. It's a bit complicated, but you can google it. Once it's done, you can host and give people your External IP Address to connect.