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

[AiChyan]

Join in progress will get you into a duty in progress if it finds one where your role is needed (in whatever dungeon/trial/roulette you chose). If it doesn't find anything in progress where your role is needed it just groups you into a fresh duty.

It can put you in a new instance even if there is an opening for your role.