It’s a private group, essentially a club, unaffiliated with either GM or the union. You pay dues to be a member.
They claim to “represent” GM retirees but they have no official standing with GM, the union, or anybody else.
Okay, got it now. They're essentially a retirement association (private and funded by dues) for the retirees who watch for legislation and lobbies for their retirees. They also offer to their retirees things similar to AARP.