I played a lot of MMORPGs that allows this very thing. Basically, every time a party member disconnects, and then reconnects while the party is still in the dungeon, they are prompted to jump right back into the same party.
Some dungeons like the Underground Graveyard or anything similar allow invitation of new members during the run. But this doesn't happen on most dungeons, especially major boss fights. It's incredibly frustrating when you're at the end of something super lengthy like Kundara or Lavalon, then get disconnected. It has happened to me a few times, but nonetheless my party members can't rejoin.
This system checks whether the disconnection is intentional or not, so no one can just logout and have coffee while the rest of the party endures the pain. Quitters can't rejoin, but those innocently booted right out of the game can jump back in.
It will be convenient for dungeons that require keys or are super long, or both.
