You can follow The Silverback Digest to receive email notices of new posts, and you are encouraged to contribute or suggest content, but beyond there is nothing (as yet) to join.
Maybe some day. Don’t send money.
In the meantime, check out this guy:
Silverbacks International will be the organization that breaks the Groucho Marx rule of “I don’t want to belong to any club that would accept me as a member.” Sorry, Groucho, but that is spoken like a true Silverback.