I don´t think this is possible yet. There is no LDAP browser built in MWG7 yet, so you have to manually add usernames or OUs to your configuration. You can use an external LDAP browser for example to get the required IDs, and add them via copy/paste.
This is not very comfortable, but the only way I can think of at the moment. I think we will have some improvements here in the future, but at the moment there is no search feature for the authentication.
Thanks Andre for your cooperation,
and how can I request for this feature to be in the near next improvemnets?