For those that haven't played Grounded, the way it works is:
– One player hosts the world during any given play session
– At the end of the session, the 'world save' is saved to the cloud
– Anyone can launch a new session at any time, and it will simply load that world save file, making you the host for the duration of that session (if you're all logging on, it's just whoever logs on first takes on the role of host).
This means you can jump on by yourself and muck around, and it will update the cloud world save, so when the next player comes on later that day they load an up to date world with all your changes.
It's basically indistinguishable from having a dedicated server, but doesn't cost anything!
I'd just add that I don't know enough about comp sci to know if there are reasons why this wouldn't be viable for Valheim, but it seems from some googling that people have been doing this already, by manually copying the world save files after every session and sending it out to their friends:
Source: https://www.reddit.com/r/valheim/comments/z39dn6/devs_it_would_be_amazing_if_valheim_had_the_same/