Guilds can be created when a person reaches a Battle Class level of 15 and has a certain amount of money. Currently, the amount of money needed is unknown and will be found out in the future. The creation of a guild is through the Guild Master NPC.
You can invite other players to the guild by typing "/guildinvite user ID" in the chat.
For the guild leader(s), they can supposedly give guild office (Basically a direct quote from the Korean site, so not too sure if this is meant as officers or guild offices, guessing the 1st). As with most guild systems, they can also have announcements for when their members login.
Withdrawing from a guild is done by clicking on the unsubscribe button in the guild window.