The Mercenaries Guild (or just Merc Guild) is the largest "Grey" faction. The players who swear alliegece to it are warriors-of-fortune (or they can be called "professional adventurers"), selling out to the highest bidder.

This will probably be a prestige class. Regardless if you join as a newbie or a vetran, you will still have to "earn your stripes" before you can hire out to player associations like Tribes or small Clans...