First you have to open portal.properties and comment the ldap.import.method=user and uncomment ldap.import.method=group. The explanation is enclosed in the comments.
# Set either user or group for import method. If set to user, portal will
# import all users and the groups associated with those users. If set to
# group, the portal import all groups and the users associated those groups.
# This value should be set based on how your LDAP server stores group
# membership information.
#
#ldap.import.method=user
ldap.import.method=group
Afterwards you have to login to Liferay as an admin user and go to the 'Enterprise Admin Portlet'-> Settings-> Authentication -> LDAP and set accordingly to your LDAP server. In every estep of the configuration you can validate that is making the connections.
Finally, section Import/Export should be configured like this:
