From Reddit: NO level sync when entering a dungeon with less that required players!

[Felis]

However, like any game they can still 'SCREW UP' to the point where the game becomes unrecoverable from a player base standpoint. I would assume that lower party members means 2-3 out of 4 or 4+ out of 8 etc. If they allowed solo content (Dungeons) at this stage and it offered experience points for new jobs etc while it might seem awesome cause you could spam the first room over and over you would be left with an empty shell of Astro, DRK and Mach Zombies running around.. not to mention a 'Zombie state of an MMO'

Non-synched = overleveled (if you can do it solo) = no exp from the enemies
It will be only for the loot

Roulette and normal PUG DF will remain to be 100% level-synched