When you initiate an expedition, the two absent players get summoned.
Works great though I think two people or even just the one with the orb would be fine as running to a dungeon doesn’t add much in the way of fun.
However, when someone leaves the dungeon early for whatever reason, the person’s replacement has no choice but to run to the dungeon. This means the people inside are stuck waiting for as long as it takes the person to get there.
There should (imo) be a mechanism for the people already inside the dungeon to summon those outside the dungeon to make it easier to replace players who have disconnected, for example.