Originally posted by loppantorkel:

It should be said that the bosses are tougher the more players are in the game, at least I believe this is the case.

This link explains enemy leveling, and scaling. It does not specify whether or not the scaling applies to bosses.

https://valheim.fandom.com/wiki/Creature_Level

Here's the section that discusses the scaling.

Creature damage is further increased by 4% and effective health is further increased by 30% for each additional player within 100 meters. These are also applied to tamed creatures. This scaling caps at 5 players.

Internally the effective health increase from additional players is implemented as a damage reduction so that it can be scaled dynamically. The reduction doesn't apply to shown damage number so it's not visible to the players. For practical purposes, it can be considered as a health increase.