After moving to Exchange 2010 from 2003 I experienced an issue design feature where a user who is configured as a group manager and allowed to update the membership list is unable to in outlook and instead receives an access denied message.
A bit of research revealed that this is the expected behaviour and is down to changes in how users can manage their own accounts and information. There is a really good explanation and work around on the exchange team blog here:
http://blogs.technet.com/b/exchange/archive/2009/11/18/3408844.aspx