I'm playing in multiplayer, and we can't seem to reliably get spirit. One time I logged in, I noticed I had 7 spirit. but I have no idea how.
I even looked through the code, it seems like you have to kill players to get it? But trying that failed.

Try kill biters

We've killed tons of biters, trees, rocks, even buildings. Nothing.

you need to be close enough to absorb their spirits
if it still doesn't work, try disabling the flying spirits.

How i can do whis?

when they die you need to be nearby (radius of 40 tiles) with a normal character (unit name: "character")

/c game.print(game.player.character.name)