The server software is included in the purchase. You can run it on either a rented/remote-hosted server or on any machine within your own LAN. As long as everyone that you want to include knows the domain name (or IP address), port number, and password for the map's server instance, they can play.

On the multiplayer tab when you start the game, there is a button specifically for entering the address:port of your target server. You do not have to go through the "official" server listings if you don't want to.