Google Apps introduced user policy management for organization.Administrators can now divide users in organizational group and control access to user group for specific application. Organizational Group Sales can have access to Gtalk while Organizational Group Development would not have access to Gtalk.
For companies which already have active directory or LDAP.Google Apps directory sync will mirror this.
Currently, this is supported from Google APPS next generation Control Panel. This feature is starting to roll out to Premier and Education edition users at no additional charge. I checked with one of domain I have with Google Apps,its not enabled for the domain.Probably, Google may change this for not Premier users.They have not said anything in their blog about it,
Official Google Enterprise Blog: Introducing user policy management for Google Apps