Guilds will be able to have their own guildhalls. Changing a house into a guildhouse is very easy. First, any player of that guild builds a house. Once the house is finished, a guild deed can be purchased from an NPC in the market area. Simply drop this deed on to the house and the house will be transferred to the guild leader. Once you do this, you give up all ownership to the house and it becomes the property of the guild and more specifically the guild leader, so make sure you clean out your vault before transferring ownership to the guild.
Guild leaders can set permissions on guild halls.
See also: Create a Guild House