Is it possible to have a multisite implementation with silverstripe that shares the users and some other data ?
Yes, there is a subsites module in subversion (which works with SilverStripe 2.1.0-rc1). There is no documentation on it that I can find though, but if you are having problem if you post here Sam or Hayden may be able to help you.
Thanks for the reply Andy. I will look over it tonight.