They were scouting for the upcoming boss fight.
It was a party of twenty members taken from five different guilds.
Ten of them reached the center of the room, then the boss appeared.
That's when the entrance closed, cutting them off from the rest of the party.

I'm sure most of you have already noticed an item missing from your main menus -- the log out button.
Let me assure you, this is not a defect in the game.
I repeat, this is not a defect.
This is how "Sword Art Online" was designed to be.

Despite my warning, the families and friends of some of the players have attempted removing the NerveGear, an unfortunate decision, to say the least.
As a result, the game now has two hundred thirteen less players than when it began.
They've been deleted from both Aincrad and the real world.