Let me start off by saying that this will only work with guilds that have a lowercase L or uppercase I in their name. Let me also say that this is only common sense. I have only found this to work with the guilds with those letters so far. You will not actually be joining the guild but this will confuse 90% of the people that look at you.
All you need to do is go to your local guild master and type in the name of the guild you are looking to join. You will then swap out the lowercase L for the uppercase I or vice versa and create the guild. It is slightly noticeable and obviously you won't show up on /who for that guild or on armory.
Example: Blades of Wrath -> BIades of Wrath
The font used by WoW makes the lowercase L look like the uppercase I but there is a slight spacing difference.
Thanks and I hope that you have fun looking like an end game raider =)