Join in progress lets you join a party in progress, but it doesn't prioritize it. You'll still go to whoever's been in the queue longer.

For in-progress, you join the queue when the party leader clicks the button to replace missing party members. It isn't based on how long you've already been in the dungeon.
This is why the roulette is very likely to just pair you with a new dungeon, since some of the dps in there are likely upwards of 20+ min wait.
If someone has queued for a specific dungeon, in-progress will also pair you with them in a new dungeon before it will let you join the party in progress, if they were queued first.