Oh, it's brilliant!!
It was my husband who found out about it, and got me the shares...
You need I think a minimum of 5 shares to join, and if you have over 20 shares membership is free - the price is quite low so it's worth going for it.
Once you have your shares you need to fill up a form (it's on their website) and send it to them. A few weeks later you get a beautiful membership card...
With it you can get lots of discounts, in the restaurants, hotels, shops... Even in the Disney Stores accross Europe!! And you get invited to special events in the park - next one is a screening for Wall-E in the studios (in French though), but they also had previews of the new rides, for example
And of course as a shareholder, you can say you own (part of) the park... I like to think that I own a small part of one of Minnie's eye lashes
You can find all the info here:http://http://corporate.disneylandparis.com/shareholders/shareholders-club/index.xhtml