The level of the guild is increased by successfully defeating raid bosses. The level of the guild determines which raid bosses are available for summoning. The higher the raid boss, the higher the possible reward (guild experience and guild coins). The guild level also determines the number of slots available in the guild shop. At the beginning only two slots are available, with guild level 10 the third slot, with guild level 20 the fourth slot and so on. The higher the guild level, the more guild buffs can be started at the same time.
Likewise, as the guild level increases, more member buffs can be acquired.