ISIS is the single most effective organization. They advocate when crises occur (largely behind the scenes), and as Algos said, advocate for "best practices". That phrase can be frustrating, because Dr. Bogduk and his Australian disciples, tend to take a very dogmatic stand. It should be pointed out that Algos is a former Board member. Mike DePalma (VCU), DJ Kennedy (Stanford), Matt Smuck (Stanford), and Josh Rittenberg (formally of Northwestern/RIC) are all current really smart, reasonable, and honest current Board members.
ASIPP is full of the worst of the worst. In my neighborhood, we have a former president of ASIPP who is amongst the biggest ****** in town. They are VERY antagonistic, and rarely play well with other organizations. They toot their own horn loudly whenever they can. The science they generate is designed to bolster our reimbursement. Quoting a study by members of their leadership is gonna get you laughed out of the room when talking to legitimate researchers (other, oddly, than the folks from Lubbock, who have hitched their wagon to Lax's team politically).
The above two organizations are mortal enemies. A few folks (Calodney for one) have been involved with both Boards.
NASS is odd. They have managed to get Ray Baker (former president of ISIS), as well as a good portion of the Northwestern/RIC mucky mucks (Akathota, Prather, Press) involved in their hierarchy. David O'Brien has also been their representative to the RUC for for several years. That being said, Carragee is the editor in chief of their journal, which should tell you all you need to know about the organization. When IDET was a promising procedure, the vast majority of members tore Kevin Pauza, one of the developers, a new one when he presented the seminal article on the subject.
The most prestigious organization no one ever hears about, btw, is ISSLS (International Society for the Study of the Lumbar Spine)