Hostan some Serious Sam Fusion survival co-op, starting with TFE Bend on the Sand, which is not pic related, but I like how it looks!

steam://joinlobby/564310/109775243258380936/76561199071486523

In theory you just copy and paste this into a browser, but I suspect that won't work, alone, so start up Serious Sam Fusion>Select "Moddable" and then copy and paste the link into a browser.

Notes:
Mid-game joining should be functional.

Using "Arcade Co-Op Survival" mod, which gives players lives based on the host's whimsy. Right now it's set to 3.

Will have to use the initiate a vote function to change maps because there's no console command to see all available maps because Croteam apparently didn't play any Half-Life game.

You shouldn't need all the workshop packages I have according to various threads on the internet. You should either automatically download everything you need on joining, or it should just werk™