Guild buffs are exactly what they sound like: buffs that go out to every member of the guild. These passive buffs to experience, health, defense and more can be earned by simply leveling the guild up. The higher the guild levels, the better the buffs become. In Mythos, guilds purchase these levels with gold.
And for guilds who are out to truly excel, Mythos now gives each group a rank depending on its guild level and how much time its members spend in the game. The best of the best will get an even better guild-wide buff to experience gain and luck. The game will hand out rankings once a week during maintenance.
You can get into the nitty-gritty details of these systems on the Mythos forums!