Hi,
If users are being removed and it is for Groups that have restriction enabled - ensure that when you add a new user in the Exchange Management Console/Shell to a Group that you also update your Restricted Group settings to ensure Active Directory then doesn't
remove that user.
I hope that helps and should resolve your issue.
Oliver Moazzezi | Exchange MVP, MCSA:M, MCITP:Exchange 2010, BA (Hons) Anim | http://www.exchange2010.com | http://www.cobweb.com | http://twitter.com/OliverMoazzezi