In LDAP there are two common ways of grouping users.  The objectClass posixGroup and the objectClass groupofNames.  What’s the difference?
posixGroup:  Uses the memberUid attribute which contains only the value of the UID of a users full DN.  For example:
cn=SomeGroup,ou=Groups,dc=example,dc=com
memberUID: someuser

groupofNames: Uses the member attribute which contains the full DN of the user.  For example:
cn=SomeGroup,ou=Groups,dc=example,dc=com
member: uid=someuser,ou=People,dc=example,dc=com

The posixGroup version is generally use in UNIX environments.
The groupof Names version is the more Windows/AD like method.  EMC VNX Unisphere expects this method.
 
Reference: http://lists.arthurdejong.org/openldap-technical/2010/02/msg00170.html