One of the most frustrating questions I had playing Tree of Savior when I reached level 50, was how to enter a dungeon queue together in a party with my friends. In other MMOs, you simply needed to start a queue with your friend(s) in your party, and the remaining slots should get filled up. Such is not the case with Tree of Savior.
Luckily after asking some questions on the forums, I got an answer. However, it is not a 100% solution, it only works 80% of the time. The key thing is to LIKE your friend and have them like you as well. There is this silly likes system in the game, where you can like someone by right clicking on them.
As illustrated by the in-game help, liking someone apparently increases your chance of meeting them in a dungeon. That means its not 100%. Some also suggest the level difference should be as close as possible for higher chance.
Personally, so far this has worked and failed a few times. But seriously, IMC Games, you need a party-finder that works also for half-full parties.
Hope this helps you.