// return fields roles, email, name, user_id
FieldsFilter fieldsFilter = new FieldsFilter().withFields("roles,email,name,user_id", true);

List<Member> membersList = managementAPI.organizations().getMembers("orgId", pageFilter, fieldsFilter)
    .execute()
    .getBody()
    .getItems();

Role firstMemberRole = membersList.get(0).getRoles();