A link instead of a server, a router config and two installs
Sven Co-op is only worth much alone, and getting a few people into the same game is historically the hard part. The traditional answers are joining a busy public server, forwarding a port on your router, or renting a dedicated one. Here is a fourth, which is what this site does.
Host a map and get a linkBrowse every co-op mapA public server is free and instant, but you get whatever map is running and whoever else is on it, which is fine for a drop-in session and useless when four friends want to play one specific campaign together.
Hosting a listen server yourself is free and gives you control, but people outside your network cannot reach it until you forward a port on your router — which is fiddly, sometimes impossible on a network you do not own, and the single most common place a co-op evening dies.
Renting a dedicated server solves both and costs money every month for something you use occasionally.
When you host a map here, the game runs in your browser and the connection for other players is brokered for you. You get a link; you send it; they open it. Nobody forwards a port, because nobody is connecting directly to your home network.
This is the same thing that makes it work on a locked-down machine — there is no software to install on either end and no router to reconfigure.
A browser and the link. That is the whole list. They do not need to own Half-Life, install the Sven Co-op client, make an account, or have played it before.
Everyone loads the same map you picked, so there is no version mismatch to sort out and no "which server are we on" confusion.
For a first session with people who have not played, pick a map the author rated easy and that has real community votes behind it. The map index lists both, so you are not guessing.
For a longer evening, the multi-part campaigns are the ones worth planning around — they are listed with the number of map files they span, which is a fair proxy for how long they take.
They reopen the link. Nothing is installed, so there is no reinstall, and rejoining is the same action as joining was the first time.