Gotta communicate mane. Tell them before you kill the last boss you need them to stay for a minute or two.
If I am tank then I always wait, and/or slow down the run if anyone asks. But the simpler solution that will give a consistent outcome is that ZOS make it so that the last person can stay in the dungeon for 15 mins or something. I realise they likely want to close all instances as soon as possible to lower resource consumption on their servers, but with the speed many people run dungeons now, they're open and closed in 5-10 mins anyway.