There is serious "broken-ness" in forming a party group while in the dungeons.
For me, it happened in the "multiplayer" starter dungeon (Sparrow?).
First, people came into the area I was in, and joined me. I have no idea if they went through a level that had been depopulated of monsters and treasures because I was there first, or if it was separately instanced up until we were joined. Considering that at no point had I passed a "point of no return", I suspect that all the rewards I had gotten up until then were denied the other players -- that actually would stink, as far as I was concerned, if I was trying to explore and kill things, only to find that someone else had gotten all the goodies first.
Second, I wasn't asked if I wanted someone to join me -- I was party leader, and could dismiss them. Had to, actually, because of lag.
Third, when I did dismiss them, well, they were in an area with an "all party members to continue" button, and I was not. With them out of the party, they disappeared from my screen, the "divider gate" went up, and I could not continue on forward.
There are some strange questions to ask about "what happens next". I'm past one "all party" button; can someone else join me now, or am I "permanently instanced" for the rest of this level? Can people only join before the first "All party" button?
Second, since I cannot continue, if I become joined, will the others that join in be unable to continue?
Third, is there any "emergency panic" thing to do at this point? What does logging out do -- am I back here next time? Am I back at the surface? (I'll find out :-).
There's a big difference between joining up with people that you want to play with, and being joined up with people that you don't want to. On a previous level, when I was at the end and had already killed off the boss, I was in a browser window reading the wiki, only to find someone had joined me, and was stuck waiting for me (I dismissed him from the party when I realized what was up). But that short time was enough to determine that this person spoke in chat speak, and was a "wave weapon first, talk later" type of person -- not the type that I'd want to party with. Equally, the two that I got teamed up with later were the "Lets just go clobber stuff" type.
* Equally problematic, I was equipped based on going in solo -- for going with a group I would need different equipment *.
Being able to join up while going through? Potentially good.
Being forced to join up? Probably not.
Finding that your dungeon instance is cleared out before you got there, when you were not trying to join a friend? Unpleasant.
All clockworks expeditions are instanced from the very beginning - there are no public areas except the towns.
If you want to keep anyone from joining you, you have to select 'solo' in the party size on the Create Party tab when entering a dungeon.
When someone joins a party, they are placed near the party leader in the current floor - previous and next floors aren't even loaded until the party as a whole transitions to the next one. It's very segmented that way.
New party members will only get distributed any loot, money, and heat that are picked up after the point at which they join.
Logging out at any point within the clockworks in any situation will return you to the nearest town.
If you wind up on the wrong side of a one-way gate and can't continue, use the Report Bug feature so the developers can fix it for future occurances.
To avoid mishaps with getting people you don't want in your party, create a party with the Friends-Only option ticked next time you're in a dungeon, and make sure the other planned party members are listed as friends in the Social menu.
All in all, beyond the one-way gate bug you encountered, everything is functioning as normal. Try looking at the game with more of an open mind, because this isn't WoW or KMMO #5748.