I'm actually looking for the same.
Something that could give a user, some sort of MEMBER ID, which should have auto-increment.
However, IN the paid subcription video uploaded on the frontpage on this site, it shows(around the 3/4 part in the video), a way to run some SQL commands, once a user is registered, maybe it is possible to somehow make a new field in that users table, and add a member_id .
But it should be done easier... Just dont know how, would love to know how, tho..