cloverjo - There are four groups of letters (3x two and 1x three consecutive letters) that are the superfluous in four across clues. They move over to four down clues. In this way all eight clues make sense (either definition or wordplay might be affected). If you read these letters in normal clue order (either across or down) you get the 'NEW MEMBER'.
The six 'illustrious members' similarly move between answers (six are added and six removed) to give 12 new entries in the grid, the 'members' are all 2 letters long.
I hope this helps, I found it quite difficult to follow the preamble (but perhaps my explanation isn't much better!).