Rent a server or set up a server on your own computer.
https://www.ionos.com/digitalguide/server/know-how/set-up-a-valheim-server/
Running a server on your own computer (provided you have good internet services) allows you and your friends to play any time you want no matter who is online when.

The problem with running it on your own computer is that it counts as a back ground program and takes up active memory and will slow other programs down and could result in lag for your friends on Valheim if you are playing something else when they are playing Valheim. A high end gaming rig can mitigate that problem especially if you have fiber optic internet service.