Here's how it happened:
* The League's Purpose: The Red-Headed League was a fake organization created by the criminal mastermind, Professor Moriarty, to manipulate Jabez Wilson. Moriarty's accomplice, the "red-headed" villain, John Clay, disguised himself as a "league" member and persuaded Wilson to join.
* The "Job": The "job" was a simple task, sitting in a small office and copying out the Encyclopedia Britannica. This kept Wilson occupied and away from his pawn shop, which was the real target.
* The Deception: The "league" was a ruse to give Clay ample time to dig a tunnel from Wilson's shop to a bank vault next door.
In short, Jabez Wilson didn't apply for the job; he was lured into it through deceit to further the criminal scheme.