You can make use of an existing LDAP directory to authenticate users to GLPI. Once you configure LDAP authentication, LDAP users will be added to the GLPI database as they log on. Some of the LDAP attributes (Name, email address, location) will be copied to the GLPI user database - with this approach GLPI is able to use more than one authentication scheme (e.g. simply use LDAP and GLPI-integrated logins together).
Parameters are slightly different depending on the LDAP implementation you are using. Microsoft Active Directory in particular uses attribute names which differ from standard LDAP attributes.