Master rank can be unlocked at skill 100 in a craft.
To obtain it, you need to craft a rare/ex item specific to the guild.
Master rank doesn't offer the ability to skill up above 100, but it does offer the following advantages:
* Cheaper materials from guild merchants: maximum price is lowered by half the difference between maximum and minimum.
* Bigger stock: items will have another stock on top of the normal stock available to all
* Access to guild store on guild holidays