If I click " join party in progress " is that an absolute that it will ?

[Garlyle]

if you choose that option, when it pops, right in the middle it'll say in Gold Progress X/X . If it doesn't it's a fresh dungeon.

Correct. You can get either a fresh dungeon or an in-progress fill depending on what's needed. And yes, it will speed up your queueing by giving you another potential way to enter the dungeon, although you might miss some chests because you weren't around for them.