"ISG encourages organizations interested in the advancement of EPUB 3 to become members of both IDPF and BISG in order to work directly with those creating the standard and developing best practices."The reality is that companies that join the two groups now are going to have virtually no influence over EPUB 3--and the specifications are public, so there's no real benefit to spending thousands, or tens of thousands, of dollars to join. (Yes, I'm suggesting being a "free rider".)
In addition, EPUB 3 is less a standard than a suggestion: The IDPF doesn't require that companies implement the complete specification, nor does it prohibit implementation of proprietary extensions--which is why Apple, Barnes & Noble and Kobo all have their own proprietary multimedia extensions to EPUB 2.X. Also, Apple hasn't endorsed EPUB 3, and isn't likely to do so.