This is not an issue ive had personally, but like rival griefing it could very much ruin new peoples experience with the game if people joining them just leave the host and do their quests on the other side of the map. thoughts ?
This has nothing to do with mission progress and the region where host is with missions. Moreover, when player has completed the tutorial missions (Archipelago region), it’s free for all, missions and regions wise.
Plethora of thoughts here, in this topic: Code of Conduct when joining a coop game
Also, one option could be turning the game into “Invite Only”, from Multiplayer settings. Since default value is “Anybody”. Though, devs have experienced with the default value and since GZ is co-op game 1st and solo game 2nd, the default value currently is “Anybody”.
Some backstory:
In the early days, the “Invite Only” option didn’t work, even when enabled and people still joined your game. I had to kick off some due to this.
At some point, the setting was fixed, so, everyone who wanted to play solo, could do that.
However, some people began talking that it’s annoying when they start the game and random people are joining them (since they didn’t know the option to toggle it) and devs set the new default to “Invite Only”.
Sure, it fixed their issue but that also in turn killed the MP lobby since unless new player specifically knew and set the MP setting to “Anybody”, no-one was joining their game.
So, another wave of complaints came and devs switched the default value around, again. Which has stood til this day.
Overall, i don’t think this issue is something devs need to fix for the players. It’s more of an issue of moral code of individual people. And i don’t think that punishing good people (e.g restricting mission progress, where only host can complete them) due to the actions of immoral people, is a way to go.
Though, there are counter-measures in the game to prevent that. Namely, host can kick anyone in their session from multiplayer menu. This has proven to be effective to keep the party at bay, while disregarding “bad apples”. 
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