Good afternoon George,
As per my research on this membership, only a handful of firms have access to this. The Financial Times went ahead and published which firms you can inquire with on gaining membership:
My suggestion would be to contact each firm directly on gaining membership.
Hope that the info provided was found helpful as if you could also take a quick second to rate my assistance so that I know my help was found useful.
Afraid that this is the most recent information provided at this time. My thought is that this list should be current and that by contacting any on the list can give you information on access to membership.
A list that was published last year I was able to locate just now. It looks like it is close to what the Financial Times has on its list: