private JSONArray getJoinersJson(Plan plan) throws JSONException {
        JSONArray joinersJson = new JSONArray();
        List<User> joiners = plan.getJoiners();
        for (User joiner : joiners) {
            JSONObject joinerJson = new JSONObject();
            joinerJson.put("uid", joiner.getUid());
            joinerJson.put("username", joiner.getUsername());
            joinersJson.put(joinerJson);
        }
        return joinersJson;
    }