gnus-harvest

This code requires that SQLite3 be installed.  Check to see if the command
"sqlite3" is already available on your system.

Once you have that, add this to your .emacs:

  (eval-after-load "gnus"
    '(progn (require 'gnus-harvest)
            (gnus-harvest-install)))

If you use message-x, you can get TAB completion of harvested address in
your To:, Cc: and From: fields by using this instead of the above:

  (eval-after-load "gnus"
    '(progn (require 'gnus-harvest)
            (gnus-harvest-install 'message-x)))