Insurgency allows anyone to host a server. Insurgency is supported by many server providers such as Vilayer. Using a server provider like this is recommended if you are not experienced in running servers.
For more advanced users, the server software can be acquired through SteamCMD which is used by many games as a way of updating dedicated servers without requiring the full Steam client. When installing and updating through SteamCMD, you will need to know the app ID which is 237410.