How to get users and groups of a organization level using API?

We can try to use query spec to query user and group from DB

For users you can refer

 QuerySpec querySpec = new QuerySpec(WTUser.class);
        SearchCondition searchCondition = new SearchCondition(WTUser.class, WTUser.NAME, SearchCondition.EQUAL, name, false);
        querySpec.appendSearchCondition(searchCondition);
        QueryResult queryResult = PersistenceHelper.manager.find(querySpec);

For groups you can refer:

QuerySpec querySpec = new QuerySpec(WTGroup.class);
        SearchCondition searchCondition = new SearchCondition(WTGroup.class, WTGroup.NAME, SearchCondition.EQUAL, groupName, false);
        querySpec.appendSearchCondition(searchCondition);
        QueryResult queryResult = PersistenceHelper.manager.find(querySpec);