Hackers Guild Membership
JoiningBuying the game is joining the guild. You can buy the game here.
Full DungeonYou can access the full dungeon once you have guild membership. On iOS and Android, the membership is per class pair (eg. both Fighters, both Good Mages, etc.), while on other platforms it is all-or-none. This allows for a lower price on mobile, especially good if you just want to play your one or two favourite characters on-the-go.
AchievementsThe game keeps track of your progress towards a number of rather hard achievements. These aren't "well done, you reached level 2"-style achievements.
Achievements are local to the computer you run the game on.
Friends & Dungeon Sharing
You can add other guild members as friends, and then invite them to play the same random dungeon via Multiplayer Gaming. You do not exist in each other's dungeon, but rather you are exploring the same dungeon independently - see who makes best use of what they find!