Guild Registrars are located in every major city. They allow you to create guilds, and, as long as your guild meets the prerequisites, change your guild emblem and increase the amount of guild members your guild can hold. Guild Registrars also allow access to the Guild Vault, if applicable.
Guild Registrars have "[Guild Registrar]" as a suffix to their names.
An easy way to find one is looking for the paper and quill mark on your map in any city.
The Guild Registrar sells Guild Licenses that allow a guild to expand beyond its default limit of 50 members.