public Set<DistributedMember> findMembers(String id, String... groups) {
    if (StringUtils.isNotBlank(id)) {
      return getAllServersAndLocators().stream().filter(m -> m.getName().equals(id))
          .collect(Collectors.toSet());
    }

    return findMembers(true, groups);
  }
if id is specified, find the member with that id, otherwise find members in the groups
geode-core/src/main/java/org/apache/geode/management/internal/configuration/validators/MemberValidator.java