Wouldn't leveling up take longer as a group? I guess it depends on how they give experience in the game. Also that lately I'm used to action games that if you are careful and patient enough, really low level characters (such as level 1, even) can take on end game bosses.
Information and resources would be huge though. Unless you got a bunch from playing the Beta I guess.
If it was a death-trap like SAO though, then the big thing would be to get a reasonably sized group of people you trust, and then keep plowing weak mobs (scaling with your level) until you are way over-leveled, bulldoze the boss and resume with weak mobs on the next floor. If dying becomes serious, make yourself invincible!
EDIT: Madd that is also cool if games let you build that in. That is also why if you are building a group you might as well get all kinds of players for balance and equip distribution.