IGG and the Dreamland Online team have produced a dev diary about the advantage of guilds in the game. In addition to the normal advantages of companionship and item sharing, guilds can also level up to gain even more bonuses.
You can even improve the guild by leveling it up. As you do so, more guild constructs can be upgraded, more members can be recruited, and more exciting guild quests with equally exciting rewards will be available. Even better, you can gain access to the EXP Tree, which grants significant amounts of EXP to guild members. There are several different guild constructs: the Guild Beacon, Quest House, Guild Tree, Guild Shop, and Guild Bank. Each grants unique bonuses that improve as the construct is upgraded to higher levels.
Read more here.