This one is actually a pretty good concept, opening a boss battle for the whole guild.
The boss can have an inmense amount of hp and guildmates could get in the dungeon in groups of 6-10 players (I don't think 8+players can be enjoyed as a game experience due to the lag that would cause).
Once everyone in the dungeon dies the run ends, with an score of the actual amount of dmg they managed to deal to the boss.
Depending on the spot you end up (and depending as well on the hp of the boss, there could be several boss levels that could be unlocked through platinum by gm or officers, like every guild starts with a 1m hp default boss, and stronger bosses with better rewards get unlocked buying "x" pack or bundle).