Examination of the governing documents is key here. The issue I would look at is why Abound users are treated differently (and in this case better?) than Legacy owners. Every property has different governing documents with quirks, but what strikes me as unusual from the description of this situation is that they are providing different amenities to owners who one would expect to hold the same deed and thus have the same amenities.
By way of example, at our property there is no legal difference between deeds held by the MVC Trust and deeds held by legacy owners. All guests, whether they come in through Abound or through a legacy owner, are entitled to exactly the same amenities. However, at our property they have historically treated Abound guests who come in through the MVC Trust deeds worse than legacy owners (the opposite of what appears to be happening at GRC). I asked Management the basis for such treatment, and that was yet another question that never received a response other than "good question - we will look into it."
My own research suggests that the legacy owners and the whole owners pressured the Management Company into treating the Abound guests as second class citizens, and the Abound guests had no way of knowing. I do believe Management was surprised to have a legacy owner standing up for the Abound users; I am not a member of Abound and have no points or any beneficial interest in the Trust, but I do believe in fiduciary duty. Board members have a duty to adhere to the governing documents and owe that duty to all members of the Association, not just similarly situated ones.
In any event, GRC must have some interesting governing documents with different classes of "owners," unless GRC is actually part residence club and part hotel per its governing documents? I cannot imagine the legacy owners haven't already looked into this issue because it is a big one, and I believe GRC has a highly competent and engaged board from the public filings I was able to review in previous (settled) litigation with the Management Company.