I would say one way is having a bunch of secondary users on your amex and have everyone use primarily that card. Everyone would then pay for their own purchases. This would only work if everyone involved is trustworthy.
The truth is though if someone has to do this and include other's spending to qualify, they probably shouldn't be spending their money on the black card in the first place. Let's face it with a $5,000 initiation fee and a $2,500 annual fee that's a big chunk of your income even if you are making 500k a year.
If it takes doing that the person may be better off spray painting their card black for $2.50 instead.
Speaking of which I often see old American Express black cards for sale on Ebay. Maybe there's a way you could buy one and re-code it with your own American Express account numbers? This has to be illegal though so I wouldn't recommend anyone do it.