in slapd.conf:
overlay dynlist
dynlist-attrset groupOfNames labeledURI member

all-users-dynamic-group.ldif:
dn: cn=allusers,ou=Group,dc=example,dc=com
cn: allusers
objectClass: groupOfNames
objectClass: labeledURIObject
objectClass: posixGroup
gidNumber: 999
member: uid=LDAP Admin,ou=System Accounts,dc=example,dc=com
labeledURI: ldap:///ou=People,dc=example,dc=com??one?(objectClass=inetOrgPerson)