This explains the SClass and Guild Master ranks and how you achieve them.
The Requirements to obtain SClass is Magic Level 20. You can fight the SClass boss to obtain SClass however if your Guild already has a Guild Master only they can promote people to SClass.
SClass' can do SClass Requests, see your guilds Sign Board for that.
The requirements to obtain Guild Master is Magic Level 30, you must already be an SClass in the Guild and the Guild can't already have a Guild Master. You must then defeat the GM Boss that only the Owner can allow you to fight.
You must have also figured out what you want to do with the Guild, what you want the Titled SClass name to be(Such as 7 Kin of Purgatory) as well as can't be a habitual trouble maker, immature brat or toxic moron.
Guild Masters can set their Guilds SClass Members and Elite Titled SClass Members. They can also edit their Guild Hall to a degree such as creating living quarters in unused space, they cannot block entrance or request boards or any npcs or anything of the like. Guild Masters can choose who in their Guild participates in the Grand Magic Games and upon winning, they will be the ones to receive the reward which they can then choose to split with their Guild.