UserQuery query = identityService.createUserQuery().memberOfGroup(groupId);
if (sortby == null || "id".equals(sortby)) {
query.orderByUserId(); // default
} else if ("firstName".equals(sortby)){
query.orderByUserFirstName();
} else if ("lastName".equals(sortby)) {
query.orderByUserLastName();
} else if ("email".equals(sortby)) {
query.orderByUserEmail();
}