What is the earliest instance of an "Adventurers' Guild" in fiction?
It doesn't have to be called an 'Adventurers' Guild', but the basic requirements are:
It's a organization that expands and has some organizational structure.
Most members of the guild have regularly fighting monsters as part of their job description, where monsters are non-sentient, hostile, fauna.
The 'Guild' owns and occupies buildings with their members, in most major cities... Like 'home bases', 'guildhalls', and 'headquarters', and 'branch office' for instance...
The Elder Scrolls' Fighters Guild and Mages Guild fits the description...
As does the Kate Daniels Mercenary Guild.