1. Import users and while importing users look at the "Group" attribute of the user and import groups.
2. Import groups and while importing groups look at the "User" attribute of the group and import all the users of that group.
By default Liferay uses the first approach.
If we want to import Groups only - not the users from the LDAP - then we need to have the 2nd approach.
To enable second approach, you need to add the following to the liferay_home/tomcat/webapps/ROOT/WEB-INF/classes/portal-ext.properties file.
Then you need to keep the "User" attribute of the group configuration blank.