Neat. If you go from a place that lets you have eight party members to a six man mission (running over land, not teleporting!), you can have all eight members play in the mission.
Arena Net rather obviously thought of this, as the only vaguely useful way you can do this is by leaving from Temple of Ages. Temple of Ages lets you have eight members in your party, but is incredibly far away from any mission.
I was just kind of running around, so I kept on going towards Sanctum Cay. When I got there, it let me enter the mission with eight characters. I keep saying characters, because it was all level 15 henchmen.
From the two screenshots I'm attaching, you can see the party standing in Sanctum Cay, showing 8/6 party members, and then right at the start of the mission (with the mission description, even).
If you didn't mind walking a lot, you could almost "reasonably" get to Temple of Ages after Lion's Arch, and then do the rest of the missions all at once with eight players. Walk from Temple of Ages back to D'Allesio Seaboard, and just go from there. (OK, if you were crazy you could go back to the first mission and go from there...)
Anyway, I was surprised to find this out, and thought I'd share.
8/6 Eight characters in six man mission!
JoDiamonds
Lost
Yes, this is how I self rush my characters to the desert. Grab some henchmen from the Temple and run to Sanctum Cay. Skips those annoying Jungle missions and boring Kryta missions.