I just made one with some fancy brackets by copying and pasting the character from the GB site's guild list, from other guilds using the symbol. So maybe that will work.
Ok, I figured out how to do it. (I just made a new guild to test, but need to give it to a spare account)
Do what Brenneh says, go to Guild > Guild List and then find guilds with the characters you want and copy/paste to your guild thing.
To find the characters you want to type into the website to search, go here:
That lists ALL the symbols, so find the ones you like. Then, copy/paste that and type it into the Guild search on Guild list (you can't just copy/paste from Shadzar's, it'll say you can't use that type of symbol).
Oh, and be sure to delete all spaces, and even without spaces, I had to redo my guild thing a few times before it worked. So if it doesn't work the first time, just keep trying and think of a new way.
You can have up to 4 alt codes.
You can thank me later >_>.