You can read all about the entrance requirements for the American Bladesmith Society here:
ABS web site
I am not so sure about calling it the 'most prestigious' knifemakers society. Many members of the ABS are also members of the Knifemakers Guild, and there are many fine knifemakers who do not belong to either organization.
The ABS deals with a specific category of knives, forged blades. It has specific performance requirements for attaining various levels of recognition (e.g. journeyman and master). It has a strong emphasis on teaching and improving the art of bladesmithing. But if a maker works primarily in stainless steels and stock-removal techniques, ABS membership is probably not going to be a goal. There are many more makers of stock-removal blades compared to the numbers of forged blade makers (smiths).
Some collectors like myself focus on forged blades, but I also have knives made by ABS Mastersmiths which are NOT forged blades. I think the ABS is a great organization which does a lot to teach and promote the art of the forged blade. If you are buying the work of an ABS Journeyman or Mastersmith, you can rest assured that the maker knows what he is doing, and is striving to perfect his art. I attach more value to the work of ABS Mastersmiths compared to other knife makers. But many of them are also members of the Knifemakers Guild.
If you talk to many knife makers, you will find that there is a lot of disagreement about the value of Guild and ABS membership. It seems that some feel the performance requirements for both organizations could stand some re-evaluation. Some of the old time members of the ABS seem to think it is getting too easy to achieve the Master smith rating. Similarly, some of the members of the Knifemakers Guild seem to feel that organization's purpose and meaning also needs some re-thinking.
There are many fine knife makers who do not belong to either organization. As a collector of knives, I attach a lot of importance to the history, integrity, and knife making philosophy of an individual maker. His or her standing or membership in a professional society like the ABS or the KG is only part how I evaluate the desireability of a makers knives. I want to know a lot more than 'are you a member of the ABS or KG?'.
Paracelsus