OpenLDAP.org hosts several public mailing lists, all of which are described at http://www.openldap.org/lists/. The two most frequented lists are openldap-software (discussions about software created as part of the OpenLDAP project) and openldap-devel (technical discussions relating to OpenLDAP development). You can subscribe to a list by sending an email to openldap-<list>-request@OpenLDAP.org, in which <list> is either software or devel, with the word "subscribe" in the body of the message.
The University of Michigan hosts a general LDAP mailing list. You can subscribe to its list by sending email to email@example.com with the word "subscribe" as the subject or by accessing the web interface found at http://listserver.itd.umich.edu/.