See the parameter $ESPCONFIG['auth_type'] and the corresponding ldap parameters in the file admin/phpESP.ini.php:

// Choose authentication type: { 'default', 'ldap_both', 'ldap_resp', 'ldap_des' }
// ldap_resp: respondents in LDAP, ldap_des: designers in LDAP
// ldap_both: both respondents and designers in LDAP
// default: mysql
$ESPCONFIG['auth_type'] = 'default';

// LDAP connection information
// (Set these values if you choose 'ldap' as the authentication type.)
// if a user is not found in ldap, the DB is still searched as well
// designer info is copied in the DB
$ESPCONFIG['ldap_server'] = 'ldap://ldap.example.com';
$ESPCONFIG['ldap_port']   = '389';
$ESPCONFIG['ldap_dn']     = 'dc=example, dc=com';
$ESPCONFIG['ldap_filter'] = 'uid=';
// the LDAP attribute that is compared with the "group" when completing private
// surveys
$ESPCONFIG['ldap_realm_attr'] = 'objectClass';
// the LDAP attribute/value needed to designate a LDAP user as a designer
$ESPCONFIG['ldap_designer_filter'] = 'UserCategory=engineer';
// the LDAP attribute needed to designate a LDAP user as a superuser
$ESPCONFIG['ldap_superuser_attr'] = 'uid';
// the LDAP attribute needed to designate a LDAP user as a superuser
$ESPCONFIG['ldap_superuser_value'] = 'test';
// most newer LDAP servers need protocol 3 to be able to bind successfully
// if this doesn't work for you, turn it of
$ESPCONFIG['ldap_force_proto_3'] = true;

// Group to add responders to via the sign-up page
// (Set to "null", without quotes, to disable the sign-up page.)
// Please do disable this for LDAP auth for respondents
$ESPCONFIG['signup_realm'] = 'auto';