Joining
From Lv. 74. Open Guild and apply. An active guild speeds up everything.
Every day
Guild Feast from Lv. 95 (lots of EXP). From Lv. 130: Guild Quests, the guild chest and the Guild Statue.
Martial Spirit
Collect shards and awaken the Guild Martial Spirit: it guards every member.
Battles
Strongest Guild once a week, Foreign Lands Treasure Hunt: hunt monsters as a guild and take their treasure.