I honestly don’t fully understand what you’re asking in the first post…
Maybe I’m just too sleepy though to properly read this.
But generally entering a dungeon with a party goes like this:
All of you queue up via the party match button, every single one (therefore everyone has to be at the dungeon site obviously). The first person in the queue actually matters. Because his level determines the level range for the other players that want to queue up. I think for most dungeons it’s +/- 20 levels (for the level 290 dungeon it’s just +/- 10 levels which is the reason why nobody queues up for it normally). So let’s say the first person is level 150, then players from level 130-170 can queue up with him. If you have two other players with the levels of 130 and 170 in the party and the 150 guy is not the first in line then you will not be able to match up due to level differences.
Anyway, once you’re all matched up you have to click the party match button again to actually get in the queue. If you have 5 people the dungeon will instantaneously open. If not, then you will have to wait for random people to get in the queue with your party.
Edit: You can always just get inside via the “enter” button as well. But then you won’t get the XP buff. That’s what people usually do for Saalus runs and the like since XP doesn’t matter with those.