For the initial membership into a Guild, one must do or have the following. (these are just first steps to get the ball rolling. Abuse them or remove them! I am not feeling that creative tonight but want to keep this moving if I can)
Stories tell of a fearsome creature who only has a single weakness, a weakness that can be exploited by a warrior of great prowess. Adventure out and find this creature. Tell us the tale of it, where it lives and how it survives. Then tell us how it can be defeated. Do this, and you shall be entered into the Fighter Guild rolls.
O person of arcane lore. Tell us a magic item that you have either created or know the tale of. Tell us as much as you can of the creation, powers and usage through the ages. Does the power have a weakness and how it can be exploited or counteracted? Tell us of this magic and you can be entered into the Magician's Guild.