This is an enhancement request for supported User & Group import/export tool.
There are some contributed User & Group import/export tool using Alfresco service API available, such as "http://tgoirand.wordpress.com/2010/04/22/exporting-importing-users-in-alfresco/.".
But, we don't have exact supported User & Group import/export tool for Alfresco Enterprise version, except for using LDAP.
That will be nice to have it as the default function.
This is not an enterprise version, but recently released version "Alfresco Team" has a function to create/import users with following steps. This could be one of the way to meet the requirement if we could implement also for the CSV export.
1. Log in Team(Share page) as admin;
2. Open Users page;
3. Press "Upload user CSV file";
4. Choose the CSV file and upload it.