There are two ways to populate users. User Synchronization Tool (outside of Admin Tool) and the User Sync in Admin tool.
User Synchronization Tool - The performance can depend on DW version and patch level. I found that the tool works way better in 7.1 than 7.0
User Sync - Go into the LDAP set section and use the test line to see what DW can see. In the test sometimes you have to check the box for "Show users of sub groups as well". If this makes your users show up when you go to the group in the test, then you will have to check the "Synchronize users in sub groups as well" box in the User Sync under the Org area.