maybe partially the answer, because why would you want thank them? they were just farming and probably didnt care whoever joined.
in the process taking away the best part of the game. being a noob and experiencing everything by your own from scratch.
Being boosted and not knowing what it even meant is a bad thing and can ruin your overall experience.
Diablo does not automatically throw you into a group with horrible people that selfishly want to steal the experience of everything being new, it puts you into game by yourself and its your own choice to either experience the game as you wish or group with others and risk the experience being different.
If you cant bring yourself to thank others for the time they spend playing with you or just to be courteous, then youre just contributing to its toxicity and should rereconsider your choice of playstyle.