If any party member leaves their party while inside the dungeon, they will be kicked out and sent back to their save point! However, they can be re invited to the party and reenter the tower. Taming any monster or killing the last monster on a floor with a mercenary will cause the instance to not register the kill, and the portal to the next floor will not open.
If a player is forced to leave the tower, they can re-enter by talking to the stone on the Misty Island again. The exit portal on the 1st floor will warp the player to the floor after the most recent boss floor they have cleared. However, the portal does not correctly warp a player to 26, 51, or 76; the player should use the Immortal Braizer instead to get to those floors.
The Endless Tower instance lasts until the party either gives up or runs out of time, after which time all players will be teleported out of the dungeon.
There is no limit on how many instances a party can create within the 4 hour limit. A new instance can be created under a different party leader if the previous instance was destroyed. Once the time limit is up, there is a 6 day 20 hour cooldown period for all party members in which they cannot enter a new Endless Tower instance. However, additional instances will not allow access to additional MVPs.
Taken from the wiki...