Your Guild can be leveled by maxing out the Guild EXP bar in your (Y) Community --> Guild tab. Guild Daily Quests can be obtained from the Guild Manager to obtain EXP when performed by members of the Guild.
Guild Level
Level 1
Member slots: 50
Guild name colour: grey
Guild EXP to level 2: 350
New skill: Adrenaline Booster
Level 2
Member slots: 60
Guild name colour: grey
Guild EXP to level 3: 7800
New skill: Moral Up
Level 3
Member slots: 70
Guild name colour: No change
Guild EXP to level: 30k+ (please edit)
New skill: Iron Wall
Level 4
Member slots: 90
Guild EXP to level: 76250 [confirmed by level 4 Guild]
New skill: maxed out, no more new skills