If "participates" in society is sufficient for "fully functioning member of society", then at a minimum, trained dolphins who perform in shows are fully functioning members of society. They have complex interactions with their trainers, including fairly high level communication, and they perform work in exchange for goods (fish).
Dolphins cannot participate in our society by agreement. They don't understand what participating is. They're captured or born in captivity and then trained to perform a few tricks. They are beautiful and fascinating, but they have no contributions to make to the fiber of our society that can't be made by other animals, machines, or even simple objects.
They are able to do nothing further. Giving them a new status is just semantics. They are fundamentally not functioning members of our society. Not individually. Not as a species.
What I would like to see is a special legal status/recognition given to dolphins. My motivation for wanting to see this done is that I think it would provide a sort of precedence for extending traditionally human rights to things that are not traditionally seen as equals of humans. I think there is a possibility that doing so may become necessary in the future, when "machines" and intelligence begin to blur.