Yes I agree that join date has a very close approximation to member number.
I was just informing him that he enabled the member number display without a way to sort by it.
If you are going to customize it, copy over the same code from the "Joined" column in to your new column. The output is the same.
But be aware, adding more columns means that you could also end up making your screen too wide for viewing on mobile devices, such as iPhones.
