WYSK: There funded by dark money PACS, but some good reporting has brought out these names: David Koch, Peter Thiel, Reid Hoffman, Mark Cuban, Harlan Crow, and Michael Bloomberg. Some of there members are most famous for stopping big bills. Joe Leiberman, for example, single handedly stopped the single payer portion of the ACA. Senators Joe Manchin and Kyrsen Simena kept the John Lewis voting rights act from passing, and famously kept the senate from repealing the filibuster.
It never looks like that from the outside because they usually talk and get to a bill they can all agree on or not. The news never reports on why bills don’t get passed beyond the parties though so the average person never sees a caucus in operation. A recent example where that veil got pierced though is the progressive caucus pushing for the green new deal and getting quashed by the other section of Democrats and all the Republicans.