Players that leave the party via the elevator->Heaven without saying a word, can really mess a party up (when the party continues on, expecting the player to continue on as well). For example, leaving via elevator-to-heaven on the end of the level prior to the boss level.... Ie leaving on the elevator that would take you to to the boss level. Party arrives on the boss level a player down and cannot invite a new player (because they are on the boss level). Infact they have no control over it and are down a player and down all the time it took getting to that boss, truely a rotten slap in the face.
Not to mention all the items the player-whom-just left, go to waste because the party does not have the opportunity to pick them up.
Maybe on the "heaven or continue on (10 energy)" screen, have a popup along the lines of "such and such as left, do you want to continue or wait?". IE allow the party to cancel the elevator and remain on that level to pickup the items dropped and fill that party slot.
If you are doing PUGs make sure everyone confirms that they are going down before a boss or else before you even start the tier confirm everyone is committed to going down to the boss. If you still have problems just make some friends and only do runs with people you trust.