thank you so much I really couldnt figure it out for the life of me! My problem was just that I didn't have an open slot in my party :/
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This is correct. you do not need an empty slot. Also, as a tip if you don't want to wait until the dungeon is lower level, just know that it will remain there even if you fail to complete the dungeon (had to retreat) and you can try again. Also rescuing them and then fleeing the dungeon still grants you the rescue and closes the dungeon, you just don't get the item the dungeon was offering.