[wp-hackers] Members Database

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I had a similar project in 2 versions. In the first i just tried to
extend wp_users and user meta... and that was a real pain. With the
second (still runing fine) i switched to the pods-plugin
(podscms.org). It gives you the possibility to handle your data in a
separate table with a nice UI and you can, if you want, later connect
the table to your wp_users extending them. As long as you don't set
the "pick-"column to required* you can easily define if someone has an
accessible account or not. Maybe that helps (as it did to me).