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General Manager Rod Fergusson clarified exactly how the co-op campaign works.
The state of the world depends on the host of the group. If you’re leveled up (meaning you’re at the same point on the questline), then you’re all moving forward. If there is an offset (for example, you invited someone new halfway through), then he will not receive progress / xp on the story quest until he catches up with you.
With this in mind, the developer advises that if you’re in a company that doesn’t constantly play together, then the player with the least progress should lead the game so they can gain quest benefits and experience.
Of course, having a group of similar strength is ideal, but this is not necessary as Blizzard has put a lot of effort into making the game enjoyable for everyone, even if your level is much lower (or higher) than your comrades.
One of my favorite features is that the game scales individually for the players. That is, if you don’t consider progress in quests, you can have level 1 and level 30 playing together and having fun because the game scales for them and the new player doesn’t have to hide in the corner.
Diablo 4’s latest OBT, or “server slam”, took place over the weekend, giving fans another chance to play the game ahead of its June 6th launch. Players were able to test their mettle against the mighty world boss Ashava, whom even Fergusson had a hard time defeating, but this is the second time a YouTuber has defeated him alone.