If one of the dev's could comment on this would be much appreciated there is no clear example I can find doing this seeing that MemberTableField::addMembershipFields is deprecated an also will not let you change the field types this would be really handy I am willing to write a tutorial on this for the wiki if I can get this working.
I've got a number of sites with this issue now, does anyone have any documentation on how to add a field to the MemberTableList now that MemberTableField::addMembershipFields has been deprecated for 2.4?????
You need to use Object::add_extension to extend the Member class and then you use the updateSummaryFields function.
There is a bit more of a detailed explanation here: