There is a thread on here somewhere with loads of info about becoming a shareholder - I'll try to summarise:
You need to purchase a minimum of 20 shares to join the shareholder's club, or you need to pay an admin fee every 2 years to rejoin. ( I don't know how much this is)
Main benefits are 15% off all table service restaurants, 10% off Earl of Sandwich, 15% off purchases in the shops, and access to Salon Mickey. There is a restriction here of 5 visits per month I think, but that's fine for most people on a short visit. Also 10% of at all European Disney Stores.
There are other discounts too - here is the official Disney page
http://corporate.disneylandparis.com/shareholders/shareholders-club/index.xhtml
You don't get an extra discount on packages, although there is a shareholder rate for hotel packages/room only, but is normally much more expensive than any package deal going.
I signed up to Halifax and purchased my shares there. I had to deposit minimum £100 to open an account and after buying shares, I was able to transfer any remaining balance back to my normal account. There are also frequent offers with Halifax where the dealing fee is waived - so you can maybe watch for that.
Once you've got them, follow the instructions on shareholder page to join the club. You'll need the contract note confirming how many shares you have, which you can get from the Halifax website. You also complete a form they provide, as they need your signature, scanned it in and emailed both to Disney.
It can take a few weeks for the Shareholder card to come through so don't leave it to the last minute.
On the meal discount alone, my shares have more than paid for themself. It's also lovely being able to pop into Salon Mickey
