It's difficult enough to find groupmates with that abomination you refer to as a "group finder tool" without being disconnected right in the middle of a dungeon, potentially missing an objective in the process.
At the very least you need to make it so that if someone in the group leaves the dungeon and rejoins, that their stage in the quest changes to match that of their teammates. This way, if someone disconnects and the rest of the group complete an objective before they rejoin, they won't be forced to repeat the entire dungeon.
Also, in the event they complete the quest while someone is absent, the missing player should be able to turn in the quest when they reconnect.
Guildmaster of Spectral Liberty - Xbox One - European Megaserver